Like Newtons' gravity, a force at a distance, how does Dark Engery cause galaxies or their surrounding space to expand from each other? But the stars within a galaxy are not affected by this expansion. Is gravity within a galaxy greater than gravity between the galaxies?

Question abour dark matter. I watched the Universe episode on this recently & had the same question your column brings up. You say dark matter is invisible, but I seem to remember that there isn't any around here. So, if we were near some, would we know it? It is detectable only by gravitational effect, but would it be visible if it was right here? And why isn't there any around here?

Dark matter is the unfortunate mathematical result of arguing against a unification of gravity and electromagnetism.  The dogmatic assertion that gravity is a constant is what creates the mystery in the first place.  Keep in mind that astrophysicists actually use a different G in their calculations for the Sun than that which they use for everything else! ...

From http://www.newscientist.com/article.ns?id=dn2814:

[excerpt]

To make mathematical models of the star's interior tally with experimental data, physicists have to use a lower value of G than is traditionally agreed. Mbelek says his calculations predict that electromagnetism would not boost gravity as much at higher temperatures, so you would expect G to be lower inside the Sun.

[...]

But other researchers are not convinced. Clifford Will, a gravity theorist at Washington University in St Louis, Missouri, believes improvements in terrestrial experiments will eventually do away with the need for explanations that rely on such exotic physics.

"In many ways it's a scandal that we don't have an agreed value for G, but if you look at the experiments, the values have been converging," he says. "In five years or so, we'll have an agreed value."

But Mbelek does not think so. Although the precision of individual measurements is improving, he says, the values are not converging."

[end excerpt]

Dark matter was first postulated when it was noticed that the spiral arms of galaxies do not rotate as they would if they were merely gravitationally attracted to the observable matter within the spiral galaxies.  More recently, we've seen evidence that spiral galaxies do not even necessarily rotate in one single direction.  I've yet to see an explanation for how dark matter solves *that* problem, although I'm sure we can expect to see more complex mathematics lacking any basis in physical processes.

Astrophysicists have become obsessed by what they can formulate consensus on, and this inevitably leads to the elevation of pseudo-pedagogical concepts that lack much in the way of scientific support.  It would be far healthier for the discipline if they relaxed their criticisms of astrophysical critics, and allowed a more free exchange of ideas within peer reviewed journals and online.  The unfortunate status quo is a burning-at-the-stake kind of treatment for anybody who professes that they do not agree with the conventional astrophysical interpretations, and this is not at all conducive to problem-solving.  The institution of the Big Bang has lost its willingness to be criticized, and they are making the common mistake of powerful institutions of pretending that they do not actually need critics and skeptics.  But the public should not be fooled.  It should be clear to everybody that if you only understand 5 - 10% of the universe's matter, then you actually don't understand the universe at all!  Ideas should not be evaluated solely on the basis of whether or not they can be discounted with overly-simplistic mathematical calculations.  Physics needs to go back to basics and ask the tough questions, like what is mass?  And what is gravity?  We all know where Einstein left off, but we should not deify his ideas if it means excluding all others from consideration.  Einstein himself wouldn't even want it that way.  The Einstein true believers have always been far more religious and dogmatic about their cosmology than was Einstein himself.

Pseudo-skepticism has become endemic within our Western culture.  The public is ascribing certainty to ideas just because they are popular.  We badly need a renewed philosophy of skepticism in our culture to evaluate what science is good and what is pseudo-.  Even though you'll rarely see it presented as such, dark matter is a huge problem for the conventional theories that badly needs to be solved.  And yet, we've already been trying to capture one of these supposed particles for decades now without any luck.  It's time for some choice within astrophysics.  If you are an astrophysicist out there reading this, please give us more choices ... Because this one is just not cutting it!

Actually, if you want to see dark matter, look all around you.  Dark matter is an umbrella term for anything that doesn't emit it's own light.  YOU and I...are technically dark matter.  Now, some may be in exotic forms, but some is common.  Your shoes, shirt, glass of beer....all dark matter.  

If string theory (or "m" theory as it has evolved) explains the universe in 11 dimensions, is it a dumb question to ask if the dark matter does not exist in our reality, but is embedded in those other dimensions we cannot experience?  Like flatlanders...

One answer for "anonymous"....of cousre we are surrounded by dark matter.  It's been shown that the only explanation for why galaxies can maintain their rotational speed and not fly apart is due to dark matter.  There is not enough observable mass in galaxies otherwise to sustain them.   The extra missing mass is assumed to be dark matter.  And we live in the Milky Way, so we are all surrounded by dark matter.   Some physicists are attempting to observe dark matter particles through deep underground measurements, but no success yet.

For the gravity question above, dark energy is not gravity.  It's a different force (unless there is someday a unified force theory derived, in which case gravity and dark energy will be part of the same force).  Dark energy is what explains the expansion of the universe, and every point in the universe is expanding from every other point, even in our galaxy.  But think of the gravity within our galaxy due to its mass as keeping it looking  stable.

I meant to answer Anonymous' question about dark matter, but a couple of Cosmic Log correspondents already have made a good start.

As noted above, dark matter was postulated because astronomers (starting with Fritz Zwicky in the '30s) noticed that their estimates of the masses of galaxies in clusters, based on their luminosity, did not match with the mass estimates based on orbital movements. They figured that there was some unseen matter that was adding to the total mass. The interactive on this page explains the whole thing.

http://www.msnbc.msn.com/id/3077833/


Everyday objects like you or that dark beer you're drinking, as far as we can tell, are *not* dark matter because we do give off light (reflected or re-radiated light counts). Even a pint of Guinness reflects some wavelengths that give it that tasty brown color.

Some scientists think that the unseen mass is in the form of things that for some reason don't radiate or reflect light - so-called massive compact halo objects, or MACHOs.

Other scientists think that dark matter comes in the form of weakly interacting massive particles, or WIMPs. These would be exotic subatomic particles, perhaps the supersymmetric "mirror" particles that could be created at CERN's Large Hadron Collider.

Right now I'm working on a project related to the LHC, and I just happen to have my notes from an interview with CERN theoretical physicist John Ellis that touched upon dark matter. Here's the question and answer:

Question: Speaking about dark matter, is it thought that these are particles that exist homogeneously in all of reality? Are there these sorts of particles in this room, or do they only exist under special conditions that you would create at the LHC?

Answer: "They exist everywhere in the universe, and not uniformly distributed. So if you go out in between the galaxies, there would be some low density of these things. But in fact galaxies were formed because these dark matter particles clumped together and then the regular visible matter, you and me, were attracted by this dark matter and formed the visible galaxies and stars that astronomers look at. So the density in this room would actually be quite high, because we're sitting inside a galaxy. What's estimated is that if you took a liter bottle of mineral water, then on average this would contain something like one dark matter particle at any one time. However, this dark matter particle is traveling quite fast. It's traveling at some fraction of the velocity of light, so it doesn't stay inside the bottle. Also, this dark matter particle has extremely weak interactions. So most of the time, it would pass straight through the bottle without leaving any trace."

That's just a taste of what we'll be talking about in our package on physics and the LHC.

Just how old would this galaxy be? I mean, how much time do the astronomers consider the dark matter would need to form this galaxy from a group of smaller ones. I think that question would give us an idea of just how weakly dark matter interacts with regular matter (IMO)

Thanks to Chris Reeve in San Fran for that post.  That is a much more articulate and informed version of what I've been thinking for years.  Dark matter and dark energy just don't pass the "smell test".  Further, as he mentioned, many scientists have become so dogmatic and narrow-minded that they dismiss out of hand anything that challenges the status quo.

I recently watched a Discovery channel program about Steven Hawking's Information Paradox, and how after many years he finally "solved" it.  The answer involved parallel universes, and seemed like a joke to me.  Perhaps Hawking's writings on the subject were not correctly reported on the Discovery channel.  In any event, it seems that physicists are just making up crap to fit their theoretical models.  It's no longer "God of the gaps", it's "math of the gaps".

thanks for hopping in and adding to this discussion Alan.  Great to see e-columnists reviewing and adding to the comments themselves.  Thanks Teach!!!

Here's one for you: dark matter and dark energy have  effects on gravity. Gravity has effects on time. Dark time anyone? A new out for the"speed of light" problem. If its truly measurable, it HAS to effect time if it has effects on gravity...Rlj

I for one am not convinced that the alleged dark matter in not in fact "slow" nuetrons that amalgamated beyond the suns which expelled them and coalasced in a sort of "L5" zone where, collectively, they defy astrophysicists whom attempt to plot nice keppelerian graphs of the orbital patterns of giant clusters, in short, no one else explained what happens to all those nuetrons our own sun generates...how much mass and energy do they represent and how far from the sun are they flung before their inertia is overcome by their surrondings. And then what happens to them?.

Chris Reeve: Why confound the physics anymore than it is already? That's not parsimonious.

Einstein has earned a right to be deified much like Newton. Didn't Einstein  derive 16 equations that describe gravity as opposed to Newton's one?

Would dark matter be what the Bible refers to as "firmament"?

A COUPLE OF THINGS TO REMEMBER
 Dark matter had a different meaning years ago. Back then it was often used for matter in the universe which did not emit enough light for us to see it at astronomical distances (invisible or undectable matter was also used but neither of any help now because "dark matter" of today is also invisible and hasn't been detected so far, only postuated to hold galaxies together). The invisible planets (only postulated back then) were considered as "dark matter" to help make up the problematic mass deficiency, but not enough mass could be found coming from them. Eventually, "dark matter" came to mean all the additional missing mass. The term (when unqualified) today means only what Alan Boyle is talking about.
 There have been some other comments in this area that suggested other stuff OUTSIDE our visible universe is supplying the "force" for our accelerated expansion picture. This conjecture is WRONG according to Einstein's Relativities because no force therein can be transmitted (or cease to act) at any speed exceeding the maxmum speed of light. (If the Sun were removed, the Earth would continue in its orbit for about 8 minutes, the light travel time from Sun to Earth, before noticing it and reacting appropriately.) Therefore, any force from outside our visible universe can't have reached us yet (can't beat light and visible universe is what light brings to us). Further, REMEMBER (explained in an earlier comment of mine on another of Alan's recent topics) that the acceleration shows up in the nearest galaxies NOT in those galaxies sufficiently far away. As a matter of fact, the astronimical observations lead to the acceleration idea because the sufficiently distant galaxies, which are seen way back in time, are observed to be moving too SLOWLY for their distance, hence, the postulated acceleration as time advances.

I agree that I think the possibility of dark matter is iffy at best. It is basically the idea that we have these equations that explained things, and as we expanded our view further and further throughout the universe with new technology, these equations suddenly couldnt explain what we saw. So we assumed our equations still worked, but we just werent seeing something. X had to be there because the laws of physics we accept to be true required it to be. But we used to think the Earth was flat, and we thought that the Earth was the center of the universe... I think there is more likely a better explanation to what we see than "dark matter" and "dark energy".

REFERENCE
 Alan's DOUBLE EINSTEIN RING with Hubble picture, near end of comments (around 80-90 out of 111?)
is where Carlton Lane of Kamuela, Hawaii called attention to often found misunderstanding of data pointing to an accelerated expansion of our universe.
Briefly, many think that the recent astronomical observations see the distant galaxies as expanding more rapidly than those nearby. This idea is WRONG. The CORRECT idea is that recent astronomical observations find the most distant (hence further back in time) galaxies expanding more slowly than those closer to us.

"Dark matter" is a term like "artificial intelligence".

So, whatever it is, is what it is. No matter how
informed we may or may not be, only our guesses,
estimates, knowledge will change. IT is constant
(as far as we know, in terms of actuality).

Guesses, arguments, observations, measurements,
theories, etc. are all just for the purpose of
gaining attention. Maybe someday we shall know.
But then, maybe never.

Regards, Pacoloco

If every point in the universe is expanding from every other point, how then can we perceive the expansion?  If my eyeball is expanding at the same rate as the galaxy, wouldn't all appear to be static?

Can thinking about dark matter and dark energy make one's head explode?  I read these posts and it was too much for me - I thought my head was going to explode...

I felt like putting on my tinfoil hat this morning and hypothesizing a different solution to the dark conundrum, so here goes!

Hypothesis: Dark matter is not a naturally occurring phenomena, it is the result of a universe populated with highly advanced civilizations operating at ~100% efficiency in capturing and recycling their em emissions.  Their usage of energy is so efficient that they capture and use nearly all available energy thereby only making their presence known through their gravitational interaction with other objects. Think Dyson spheres with Harry potter cloaks.  Civilizations using this technology would disappear from anyone (us) trying to make observations of the electromagnetic spectrum, but would remain visible to anyone (us again) performing measurements of gravity and it's interaction on celestial objects.

This also explains why after half a century of increasingly sophisticated observations of the electromagnetic spectrum we've continually failed to detect any other civilizations.  

Any hypothesis should be testable, and of course this one really isn't.  However there could be some circumstantial evidence that could be evaluated to support this idea.  Does dark matter tend to clump anywhere?  IE is there more of it around the periphery of galaxies with active nuclei?  Is there more of it in older galaxies implying more civilizations there?  Do the oldest galaxies visible show a lack of dark matter (IE galaxies old enough that not much life would have evolved yet at the point we are viewing them).  Would it be theoretically possible for a civilization to build something to make a solar system resemble dark matter?

Heh, there wasn't much good pseudo science in this thread yet so I felt the need to contribute!  Enjoy.

"Actually, if you want to see dark matter, look all around you.  Dark matter is an umbrella term for anything that doesn't emit it's own light.  YOU and I...are technically dark matter."

Not true.  Dark matter is that which does not emit OR REFLECT light.  If you and I couldn't relflect light, we wouldn't be able to see each other!

That's why it's called "dark" matter, because you can't see it at visible wavelengths.

This story is further proof that light itself is both dark matter and dark energy.  You cannot see light as it passes through space because it is only visible when it strikes an object no matter how big or how small.  Since light consists of particles (there are no massless particles), and, since it is traveling at the speed of light (duh), it gives up tremendous energy on impact which is why the universe is expanding at an ever-increasing rate.

cant the gravitational lensing be explained if i were to consider that dark matter is made up of extremely small non-baryonic particles (WIMPs)??

So if the visible matter that we are all familiar with has energy which is expressed in E=MC^2, then could dark matter have a different relationship to the speed of light other than being squared?

Jerome asked:

"But the stars within a galaxy are not affected by this expansion. Is gravity within a galaxy greater than gravity between the galaxies?"

Yes, depending on what you mean. The gravitational force (so far as we know) inside a galaxy behaves the same way as that outside a galaxy, but it can be weaker or strong depending on what's nearby. The stars, and dark matter, in a galaxy hold the galaxy together even as the Universe as a whole expands.

Chris (and Tod),

It may be that astrophysicists are rejecting good ideas offered by others - it happens, but you haven't made a case yet. The idea that astrophysicists are unwilling to embrace new and controversial ideas and are beholden to some orthodox is crazy. Dark matter and dark energy both began as unorthodox challenges themselves!

Both of these ideas where challenged and had to earn their place. I don't recall proponents whining about this. It's part of the process of science. There were complaints about the observational data, for example, and these had to be addressed.

One problem is that it's fairly *easy* to come up with a simpler theory that explains some aspect of the problem - for example, you don't need dark matter to explain velocity distributions in a galaxy if you use some alternate theory of gravity. The trouble is that you then are obligated to use this new theory of gravity to explain *everything* else we see in the universe. I think astrophysicists aren't willing to give much time to other ideas that don't attempt to meet this criterion.

As an example, dark matter and dark energy aren't just used to explain galactic velocity distributions and universe expansion. They also fit in very well with Cosmic Microwave Background measurements. These are measurements of the last light from the early universe when atoms first formed. And after a proposed new theory of gravity addresses those measurements, how about abundances of nuclei from the Big Bang? Star and galaxy formation? The Big Bang theory with dark matter and dark energy do a decent job.

well jerome i may be able to answer you yes, intergalactic grvity is stronger because like einstins theory of relitivity states the universe is like a wrinkled sheet but when a mass (say a galaxy) is placed on that sheet the sheet is depressed and smoothed "creating" gravity. as the distance greatens the masses effect on the sheet of the universe is lessend

"As a matter of fact, the astronimical observations lead to the acceleration idea because the sufficiently distant galaxies, which are seen way back in time, are observed to be moving too SLOWLY for their distance, hence, the postulated acceleration as time advances."

This may be simplistic and "noobish" of me, a layperson, to ask... but would the notion of the speed of light NOT being constant over the lifespan of the universe also explain why distant galaxies appear to be moving away from us more slowly than closer galaxies whose movement is measured from a more recent timeframe?

A distant galaxy with less red-shift than a closer galaxy... means that it's moving away more slowly?  What if it means that light leaving that distant galaxy traveled more quickly than light emitted from a closer source more recently?  Would slower light from a closer source moving away from us look like it had a higher red-shift than light traveling faster from farther away?

Is there any possibility that light has slowed down a little over time?  Has anyone looked into this possibility?

Don't flame me!  I'm a noob!  *cowers*

actually the universe we can see with our eyes and telescopes is just a tiny thread in the fabric of space and time!

Dear "Dummy with a question",

Your eye isn't expanding - it's held together by strong electromagnetic forces. The galaxy isn't either - it's held together by the gravitational forces between stars, gas and dark matter in close proximity.

P.S. You should change your name. That was an insightful question.

For an explanation of the universe that does not need dark matter or other hypotheticals, see website thunderbolts.info, read the archives, and keep an open mind.  Plasma and the electromagnetic force can achieve all the postulated phenomena, one of which is the anomalous rotation of spiral galaxies.  I am not affiliated with thunderbolts.  These people have made  predictions at solar system and cosmic scales; at the planetary scale they have scored predictive successes (for instance with the idea that comets have asteroidal compositions, now confirmed by the NASA Stardust mission.    Cheers

To Dummy with a question: try and Google "red shift" that may help. Hasn't anyone thought of branes? Could it be possible that gravity is leaking either into or out of our universe through close proximity to other branes?

RE:

"Einstein has earned a right to be deified much like Newton. Didn't Einstein derive 16 equations that describe gravity as opposed to Newton's one?"

It wasn't until late in Einstein's life, or thereafter, that we came to discover that the interstellar medium is not a vacuum at all, but is instead filled with charged particles.  And yet, this dramatic discovery was rolled into the current paradigm rather than challenging it.  Kristian Birkeland, the world's first laboratory astrophysicist, who operated around the same time, theorized a different possibility: that these charged particles that fill space must actually do things of importance.  There are huge implications to the fact that we established consensus on the current conventional wisdom *before* we fully quantified the true composition of space.  The fact of dark matter's significant role in astrophysical theories today is a clear indication, if you ask me, that Einstein might not have been leading us in the correct direction.  If we are to avoid treating science as a religion -- which requires that we apply skepticism in a fair manner to everything that scientists are telling us -- then we should at least reserve judgment and encourage competing theories to be heard out before allowing this prior consensus to continue to stand.  Understanding only 5% of the universe quite clearly demands that alternative ideas must be heard out so that we have something to compare the conventional theories against.

Kristian Birkeland's laboratory studies of the aurora continue to be validated to this day.  We'd be very smart as a culture to collectively revisit his works in their entirety, especially considering that he actually predicted back in the early 1900's that dark matter should exist in the form of "corpuscles" or "pencils" of electromagnetic charges.  Birkeland was doing nothing more than applying Maxwell's Equations to space phenomenon in a pretty straightforward manner.  The problem is that astrophysics has set itself at the top of a scientific hierarchy where people like Birkeland today -- who thrive on laboratory study of plasmas -- are not always considered to have opinions worth hearing out within the astrophysical community.  When I try to talk to people online about these issues, based upon my own conversations with laboratory plasma physicists, I'm constantly told that there is no actual debate worth discussing.  There is little attempt by the mainstream to actually understand things outside of the context of the conventional paradigms.  In fact, most of them are completely unable to evaluate cosmologies in a fresh manner; they consistently view competing cosmologies through the lens of what they've been taught about the conventional paradigm.  And yet, some of the fundamental debates within cosmology remain essentially unsolved.  So, the real question is: if dark matter persists in being dark to us for decades now, wouldn't we be smart to finally start exploring alternative cosmologies?  It's a legitimate question.

The public is under this impression that the Big Bang and all of those supporting theories are the *only* possible cosmology that we can develop.  What they don't realize is that this is completely untrue.  It's just that it's where the money's at.  We can quantify a fair number of cosmological ideas to the same extent that the Big Bang is quantified if we want to.  It's up to the public to decide that these theories are not going anywhere real fast, and exert pressure on NASA and the astrophysical institutions to create competition within their discipline.  If we continue to be apathetic and not dig into what these guys are saying deep enough to actually formulate meaningful criticisms for these guys, then we are squandering our singular opportunity to develop innovative technologies within our lifetimes that will improve our ways of life.  Understanding astrophysics is one of the few keys available to us at the moment for unlocking the greatest mysteries of the universe.  Unfortunately for space journalists, they are on the front lines of this lack of skepticism, and if there in fact is a dramatic paradigm change in our future (as I believe there will be), the credit will go to those few journalists who were willing to defy the ridicule of professional astrophysicists and distill the "fringe" ideas (like the Electric Universe) into a new context that the public can see the logic within.  There are amazing pressures on these journalists to cow to the dominant paradigm.  We're currently waiting for somebody to step forward and challenge this system.  Once it becomes acceptable to challenge it, you and others will come to see that evidence can be and has been amassed to support multiple cosmologies, and Einstein's work is just one of many for our consideration.  Popularity has a tendency to possess an aura of invincibility and singularity, but in fact, these things are illusions that we grant to it when we include bias in our own skepticism towards "fringe" ideas.  In the real history of science, many past "fringe" ideas have turned out to in fact be true, and even in spite of the strong appearance that the problem was already solved.  For scientists, I would refer them to the two-fluids "action at a distance" paradigm that Faraday and Maxwell had to bust in order to bring us a new theory of electromagnetism -- a theory which has essentially heralded most of the amazing era of technology that we are witnessing today.  Had we not been willing to consider that we might be wrong at *that* time, we would have paid a price in terms of the technology we have access to today.  It's time for us to see if a similar re-evaluation of the fundamental forces within cosmology could herald in another new paradigm.

Well, IT looks like we have come to a consensus.... what we "PERCEIVE" may not "EXACTLY" be what "IT" is.

I'm enamored by the assertion that "WE" really don't know what "GRAVITY" really is!!!

Do "WE" really know what happened before the "EVENT HORIZON"????  Did the Universe expand at a rate that we could/can never comprehend? Think about that my friends!

Will someone... "ANYONE"... in the Scientific Community "Step up to the Plate" and tell all of us what we are missing here.

WHAT! no one got the "BIG BRASS ONES" to "STEP UP TO THE PLATE!" and tell the rest of us Morons "WHAT" we NEED TO KNOW?

HUH! HOW PASSIONATELY FEEBLE MANKIND REALLY IS!

Someone please give me a break! If I could just change my career and be an Astrophysist, jezz I would have it made in the SHADE!

I'm no Einstein but even I know what can be is "ALL" that can be and what "IS" is all that can be. From that we progress to the "Reality" of what we are.

Are there 11 or an Infinite number of deminsions? Let's "STOP" mucking around and "LETS ALL" get together and figure out "WHAT" "IT" "IS" and get on with "IT"............ LOLFOF ;-)

Sorry...... I have a passion, if you do not, please excuse yourself and move on ;-)

Sincerely, TommyTrouble AKA Infinitum@cox.net

In reference to the thoughts expressed about the impermeability of today's paradigms to outside ideas I would like to say that currently I maintain several "working" paradigms that cannot interact. I know that none of them are sufficient to explain everything but over time they seem to reach for each other in subtle ways so I must be very fluid in my thinking. Recently I've come to think of the dark matter as the outside of an Oreo cookie and dark energy as the mysterious force that twists it off to get at the visible (white) matter inside. Believe it or not, it's as good a model as any other.

Why not consider the old concept of ether for explaining dark matter, this hipothetic strange substance supposedly had strange properties like have a large density, have the abilty to hold the bodies surrounding them with its force,etc. That would explain why the galaxies and all that exists is compact and concrete and also it would explain why the universe expands because of its intrinsic property which is fill out all the space or the emptyness of the universe pushing all the bodies out of each other if we think that our universe has the possibility of being infinite. In this way at close range this substance would act as negative pressure and at larger distances like positive pressure. The fact that thsi substance could not be explained because of not disposing of a method of detecting it doesnot mean that not exist and it would explain many other things also.

I've gone back and forth a bit about my feelings on dark matter and dark energy.  I used to think the postulation of them was a poor attempt to make sense of what we were observing based on what we already knew.  It seemed as if it was a way of describing things in a manner that appeared scientific, while ultimately meaning nothing more than "we don't know what's going on out there."
Now, I realize I can accept all that as true without a problem, so long as we don't become complacent with our current understanding of the universe, and as long as we continue to try to understand it better, and more specifically.
All the experiments conducted thusfar to directly observe the stuff of dark matter have failed. We know it's supposed to be hard to detect, so we wouldn't expect it to reveal itself easily to us on the first try, but at what point do we eventually concede that all testing does NOT support the existence of dark matter?
Hopefully more about the universe will soon be understood with what's going on with the LHC.  Maybe it will finally reveal to us the final puzzle pieces behind the couch to complete our picture of the universe.  However, if we don't soon get some good observational evidence for the existence of dark matter, I think it will be high time to start spending a lot more time, money, and energy (the non-dark variety) on the development of alternate theories to explain the observed orbital anomalies.
I strongly feel that other factors could be responsible for the strange gravitational effects we've seen.  Our understanding of both space and gravity may need to be expanded to make sense of such things, and someday we maybe able to do so utilizing forces we alreay know exist, operating in way we don't yet understand.
Good luck astrophysicists of the future!

i think, therefore i provide a way for  Universities and tax payers to provide me income.  Because i can neither be proved or disproved so others can eventualy take my place.

Isn't it fun to compare theories?  Even we who are not scientists have them.  We don't know if what we 'see' is dark energy or just ionized matter.  It's fun to try to understand it all, though.  Dark energy is a THEORY that has been difficult to prove.  Light is typically seen and other 'stuff' isn't.  How can we possibly see all matter by looking at stars?  Stars are only an indicator as they represent such a small amout of existing matter as to be almost insignificant.  We cannot even see most distant stars before they becomes type 1A supernovas until after they explodes.  To us they is an invisible stars 'till then.  How can we count them until we see them?  There is no way we can see stars or a galaxy and say we see all matter there.  We have a lot to learn and theories are just that.  Still, it is fun to hear and compare theories.  Even more enjoyable to compare what is 'learned'.  Theories are correct until proven wrong anyway.  Einstein even thought he was right, then wrong, then was proven right in many things.  Science is just: 'We think we see this and we think this means that'.  Kool but frustrating too.

ok i am not a scientist of any sort but i was wondering... among all the things that we don't understand in the universe couldn't dark matter be something that we don't have any real understanding of!!for example TIME!! what is time and if time here can be differant than say time in a black whole couldn't time also be differant in other places?
or could time be something that is so differant than what it is thought to be? maybe time is a exotic partical (like dark matter) that passes throught everything and in so doing affects everything causing what we view as aging or decaying of phyical matter. and if this was the case it would explain why time travel is so far fetched and why black holes have a differant time dialation( maybe the gravity of said black hole repells these (theoredical) time particals?

if this is just all crazy talk i appologize for the waste of time im just very curious of the whole subject of space and time!!

My MACHO can beat up your WIMP.

Dark matter can only be observed by it's gravitational affects.  It has mass, but does not interact with normal matter besides gravitational affects.  We have maps of dark matter and have concluded that indeed dark matter exists.

It powers my Tie-Fighter.

Wasn't it Newton who postulated that for every force there is an equal and opposite reaction. If the Universe program was correct then dark matter is not an equal and opposite reaction to gravity. Is there something wrong with this?

The recent observation that the universe appears to be accelerating its expansion rate as time goes on seems surprising in that one would expect quite the opposite, that gravity would slow the expansion over time rather than accelerate it. Thus the idea of dark energy pushing the universe apart was proposed in order to explain this. There is, however, another explanation for the observed apparent accelerating expansion that does not require postulating a new undetectable form of energy, and that is that the passage of time itself is accelerating. If one graphs the expansion rate over time as one "unit" of expansion rate per one "unit" of time, the result, assuming an unchanging expansion rate, would be a nice straight line, the slope of which giving the rate of expansion. But if the time units are decreasing in size (such as if one plots the same graph using logarithmic graph paper) then the line becomes a curve with the slope of the line ever increasing and makes the rate over expansion appear to be slower (i.e. a small slope) in the distant past. The idea that time is accelerating is commonly experienced subjectively (I know I seem to have less and less time available to me) and may be related to the increase in the Schumann Resonance observed over the last two decades (this connection being beyond the scope of our discussion here), but this may be the first evidence that the phenomenon may be universal and not just a local effect.

To the Village Geek and Others
Including "Stephan Hawking"
 The most distant galaxies do NOT have less red-shift than those closer in. The distant galaxies DO have a GREATER red-shift than those closer in, BUT recent astronomical observations seem to show that it is NOT AS MUCH GREATER as it would be if the expansion were not accelerating (velocity of expansion is greater "now", in our vicinity, than it was in ancient past, vicinity of very distant galaxies as seen by us),
 "Hawking's" oreo (quotes because not sure comment came from him due to physical problems of "LAYERED" Oreo and many other possible misconceptions; besides, I feel he is more clever and kind enough to avoid such things, but it could pass as some of his humor?) model (analogy?) is NOT as good as any other. Maybe as "valid" in view of our current degree of probability, but far from as good. Besides, is it God who does the twisting?
 Finally, our universe seems to be of such a nature that we can uncover some of its ways (Einstein, Hawking, and many others found great amazment in how well mathematics fits and is able to help so much in the unccvering). Many suggestions in the comments would result in making our universe guilty of great deception of us or requiring unnecessrily complicated explanations especially where simplicity (but NOT oreo cookies) is closer to the truth. Think of the tangled webs that can arise from lying (guess the devil, "great deciever", believers might see this as exactly what they are trying to tell us). Deception of such magnitude flies in the face of the self-consistency we find and RELY ON (more self-deception?) in our universe. Oftentimes, we do decieve ourselves, but our universe seems to get us back on the right track, eventually, even if some troublesome times may occur along the way. TRUTH, we hope, and our universe seems to agree, will out in the end (guess the God believers might see this as exactly what they are trying to tell us).

Carlton Lane:

My apologies, I had my red-shifting backward.  Larger red-shift for more distant galaxies.  Okay... I've got it.  But hey, I was a Liberal Arts major, so I have a good excuse.  So thank you very much for fixing my model!  But... you didn't break the question behind it.  It doesn't speak to the possibility that C isn't as constant as we think, does it?  

After all, which is harder to do: accept undetectable matter and energy, or say that some guy's equation is wrong?

While I find your reply intriguing and I want to understand more, I'm afraid you lost me.  Is there any chance you can reply again and maybe clarify a little?

To The Village Geek and Others
 There is other astronomical
stuff that does not depend on "c", the maximum speed of light which is a number that arises from Maxwell's electromagnetic equations, which confirms data that does depend on "c" and its being a constant. This agreement of diverse observations provides convincing evidence for the constancy of "c". Maybe, billions (trillions?) of years from now "c" will be slightly different; meanwhile, our current use of "c" as a constant should take care of us for some time.
There are other reasons and experimental results that find "c" to be a constant and, if it were not great complications arise. The often heard "subtle is the Lord, BUT NOT MALICIOUS" of Einstein is pointing out how our universe has revealed subtlety but not malice. A "c" that varied significantly with time would be VERY malicious. There are other constants of physics, such as G, Newton's gravitational constant, e, the electronic charge, h, Planck's constant, etc. that people have toyed with by positing that they vary over long periods of time. Most lead to unnecessary complications (malice on part of universe?) and none have found any supporting data.
 The similarity to the way that Einstein's gravitation CONFIRMED Newton's gravitation (even found Newton's "G", gravitational constant) and confirmed that Newton's gravitation is sufficiently accurate for us getting around in our solar system [and other similar systems as long as GM/(Rc) is kept sufficienty small compared to 1, where M is the mass involved at distance R] is most likely what we will find if "c" is ever discovered to vary over long time periods (billions or trillions of years); specifically, our theories that take "c" as constant for long times are sufficiently adequate for the times rhar are currently realistic for us. Again, if one feels that our universe is of such a nature that we may uncover its truths (laws?), then variations in c, G, e, h, etc. must not be significant over our time (thereby, avoiding malice?). A variation in "h" could be postulated as the cause for the red-shift, but the complications introduced in explaining quantum observations and "miracles" are unacceptable (would make universe very malicious). Taking a variation in "c" as the cause of the red-shift is even more unacceptable for the havoc it would cause in trying to uncover the facts of our observable universe.
 Finally, there are many who want our speeds in our universe to be able to exceed "c" (the maximum speed of light, Maxwell's electromagnetic number), but such a situation would not only make our physical universe MALICIOUS; but, even worse, it would make it inconsistent or self-contradictory (hence, make it impossible to uncover and enjoy its truths). Our physical universe could be seen as MALICIOUS had the value for "c" been appreciably different from that which it is. For the religious, God knew what He was doing and any complaint might get us cast out of Eden like Adam and Eve for complaining that they knew better than God. For the non-believers, do you think that you could have gotten it better? You may say so, but there has to be DOUBT (non-belief) in your heart about that too. How about all the politicians telling us that they will do it better. Well "better", maybe, but better than the universe does?
Many will say they would have made the universe without the possibility for such harm we see, but to do so they would have to remove free will. Anyone going to vote for a canddate that will do away with free will?

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