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Is dark matter really a form of matter? We know it has a gravitional attraction and we know that everything that has a gravitional force is made by matter. But maybe this is an exception?

And if there are such theories, would there be theories on that dark energy is not really energy?

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Anything that interacts gravitationally is matter by definition. –  Ron Maimon Apr 2 '12 at 6:07
    
Dark matter is just like normal matter, except that its dark. MACHOS are just normal everyday stuff, the could be made of interstellar waffles for what it's worth. WIMPS are a bit stranger, maybe neutrinos (These are everywhere, and they're matter--just that we don't really interact with them). On the other hand, Dark energy is a rather strange concept. It is sort-of-kind-of energy, but it's different. I would have linked to an image of $\lambda$ blowing up the universe which I'd seen somewhere; but I can't find it :/ –  Manishearth Apr 2 '12 at 6:35
    
@RonMaimon - is that a good definition? Is a photon "matter" ? –  Martin Beckett Apr 2 '12 at 16:29
    
@MartinBeckett: yes and yes. A graviton is matter too, even though at no local freefalling frame is there any stress energy. –  Ron Maimon Apr 2 '12 at 16:36
    
@Manishearth: but dark matter does not emit photons, right? Does dark matter absorb photons? If it is the gravitational force that absorb the photons, why cannot we see the dark matter? –  Rox Apr 2 '12 at 17:06
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Yes, dark matter is really matter. Specifically, it seems that dark matter consist of massive particles that "clump" around galaxies. This can be seen via gravitational lensing (and other techniques) which allow one to form a map of the gravitational being produced in some region. Once one subtracts of the gravitational field of all the non-dark matter constituents, the remainder is presumed to be from dark matter.

A note on the definition of matter. "Matter", as opposed to a "force carrier", is generally taken to be something massive, and also anything comprised of spin 1/2 particles. Thus, neutrinos were considered to be "matter" even when it seemed that they might be massless. Everything interacts gravitationally, so this won't suffice to distinguish "matter" from forces. Really, one might argue that the distinction is ad hoc anyway. In any case, one doesn't consider a graviton itself to be "matter" (even though it does interact with other gravitons) nor is a photon considered to be matter, even though it can produce and be affected by gravitational fields.

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it's not just "any" matter!! you may call them WIMP..but are they really matter?? the only thing with which they interact is gravity AFAIK... –  Vineet Menon Jun 29 '12 at 12:34
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Dark matter blob confounds experts
A STUDY OF THE DARK CORE IN A520 WITH HUBBLE SPACE TELESCOPE: THE MYSTERY DEEPENS and arxiv Feb,28

.. is at odds with what has been observed in other ..

The quest to find DM exist for many decades unsucessfully, and it's existence rooted deeply in our heads. I remind that DM and DE are, in their inception, hypotetical entities. It seems to be be forgotten by all.

There are other theories because DM & DE are subject of intense speculation.
But all of them appeal to more untested, if testable, strange entities, like multiverses or other dimensions, or MACHOS, WIMPs, etc.
The physical tests for detection had null results, inspite of, according to prevalecent theory, we have to be immersed in DM & DE.
If DM exists then why the Solar System has no trace of it? The Sun should have grabbed some of it to it's surroundings. But there is not a single clue that DM exists (i.e forming a gradient in mass) in the Solar System.

In this PSE-post I explain why I'm confident that there is no need of DM nor DE to explain the universe evolution.

In galactodinamycs there is more to consider than a central force, for instance:
find where are the neibourough voids wrt the Milky Way (*), and also conclude that the voids are inevitably growing.

(*) I'd appreciate if anyone around intends to check that they are in the opposite direction wrt to the direction of the Great Atractor, as I beleive, or whatever other direction. Any reference to data that shows that the voids are growing is appreciated. Downvote with quality, find papers with data.

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