As this question, or something close to it, comes up often, in various sections of BAUT, I thought it would be a good idea to have a thread devoted to it.

First, some words on scope. This thread is primarily about the sets of observations, done by astronomers and others, which have lead to the idea of dark matter being fully accepted by the astronomical community. Of necessity, some theory will also need to be covered, particularly General Relativity and Newtonian dynamics.

Two classes of 'dark matter' are somewhat tangential to this thread - 'hot dark matter' ('dark matter' which is, or was, moving at relativistic speeds; neutrinos are an example), and 'baryonic dark matter' (or 'ordinary matter' - atoms and molecules and ions of H, He, ..., whether in the form of plasma, gas, dust, or bigger clumps that does not emit detectable electromagnetic radiation; this also includes baryonic matter in the form of white dwarfs or neutron stars).

Recently, a good, popular-level book on DM has been published by Springer-Praxis "In Search of Dark Matter", Ken Freeman and Geoff McNamara (ISBN 0-387-27616-5).

First of all, CDM was proposed to explain the rotation curves of spiral galaxies.

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T. Anderson

CDM is needed for a bunch of reasons, this is an incomplete list:
1) to get the gravity right in spiral galaxies, as Kaptain K already pointed out
2) to get the gravity right in clusters of galaxies, as inferred from their motions
3) to get enough gravity for 1 and 2 above without messing up the observed fractions of light nuclei that were formed early in the Big Bang
4) to get a flat universe (in comoving coordinates) as expected from inflation. Even if you don't accept inflation, the fact that it is near flat would be spectacularly suprising if it wasn't virtually exactly flat.
5) to explain how galaxies were able to form from a nearly homogeneous matter distribution (baryonic matter alone could not have caused galaxies to form so quickly).
6) to avoid altering the theory of gravity, a very successful theory with excellent axiomatic underpinnings, with something completely ad hoc that so far has not proved to be promising.
7) to allow the possibility that not all matter interacts with light, which otherwise would be a very photon-ocentric view of the universe just because we have eyes.

Non-baryonic dark matter is needed due to, for instance, graviational lensing surveys show that baryonic MACHOs aren't numerous enough, and other studies finding that baryonic WIMPs aren't numerous enough to account for the necessary DM to agree with Ken G's list above.

I don't understand it well enough to explain it -- perhaps someone can step in here with a clear explanation -- but apparently specific information can be drawn from careful measurements of the CMBR to enable the following conclusion:
This statement comes from the Conclusion section of the Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology.

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Nereid said, "'hot dark matter' ('dark matter' which is, or was, moving at relativistic speeds; neutrinos are an example)"

I was under the impression that neutrinos have mass, is that correct? (and do we know how much mass? As much as an electron? Less?) Assuming that they do have mass, I must infer that by "relativistic speeds" you mean speeds approaching, but not reaching, c. Is that correct? I just want to make sure I`m clear on this...

Maybe, maybe not. The latest data strongly indicate that (at least) two of the three "flavors" of neutrinos may have mass.
Very much less. Data so far only set upper limits and these limits are a very small fraction of the mass of an electron.
Yes.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

This is probably a bizarre question, and I`m not trying to go off into woo-woo-land, but maybe someone could help me out with the following (and of course, disabuse me of any misconceptions I may be harboring). Photons have no mass, and travel at c, which is the fastest possible speed. Objects with mass are prohibited from ever reaching c, but would any massless particle (assuming others are found) necessarily move at c, or could massless particles possibly move slower in their own frame? (I understand that photons can be made to go slower than c in our frame, but in their own, they are still moving at c, right?) If so, do we have any idea why some massless particles would move at different speeds than others?

On a related note, does anyone know why c is what it is, and not faster or slower? Why that specific speed?

What's "the great attractor" ? Can this be explained in terms of non-baryonic DM ?

The great attractor is probably a very massive galaxy cluster located where we can't see it behind Milky Way dust clouds. It is the explanation for why relatively local galaxies are having their velocities changed in that general direction.

There isn't a specific physical reason that it couldn't be explained by a large collection of purely non-baryonic dark matter, but so far we haven't ever seen something like that, so the odds seem pretty low.

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All massless particles move at c in a vacuum, relative to any observer. Were this not so, contradictions with the fundamental postulates of relativity would appear. Maybe those postulates are wong, but they haven't let us down yet. Also note that it isn't true that photons move at c in their own frame, anything is at rest in its own frame if it has a frame, which massless particles do not (no observations can be made from the frame of a photon, so it is not an allowable frame for doing physics). Finally, the question of why c is what it is is very profound, and I have no idea the answer, except to say that there are basically three possibilities. Either c=infinite, which would do violence with the principle of cause and effect (causes and effects could happen at the same time), or c=0, which would not allow anything to cause anything else, or c is somewhere in between, which allows for an interesting universe that acts like ours. The value of "somewhere in between" seems pretty arbitrary, but it is important that it is a very fast speed relative to our daily experience. Were this not so, even cavemen would have attained some level of understanding of relativity. Why is it such a fast speed relative to our experience? I've no idea.

<<The great attractor is probably a very massive galaxy cluster located where we can't see it behind Milky Way dust clouds. >>

Where I was coming from was, if there's some unexplained huge mass in intergalactic space that we can't see, and if that is being explained by baryonic matter, how do we know in the depths of the universe there aren't super-great attractors, or indeed large numbers of difficult-to-detect minor great attractors?

However, you seem to be saying it's just a line of sight problem, if we could see past our own milky way dust clouds, we'd be able to see it.

Has anyone checked through distant galactic vectors to check for other great attractors?

The problem is that for great distances, all we have are radial (line of sight) vectors. Any translational (perpendicular to line of sight) vectors are to small to show up, given our instrument accuracy and time of measurement.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

KenG said, "Also note that it isn't true that photons move at c in their own frame, anything is at rest in its own frame if it has a frame, which massless particles do not (no observations can be made from the frame of a photon, so it is not an allowable frame for doing physics)."

d`oh. Thanks, I`m trying to learn to use the terminology correctly.

See the tutorial "Anisotrpies in the CMB", from Wayne Hu. The basic idea is that the height of the 2nd acoustic peak, relative to the first acoustic peak, depends on the scattering of CMB photons due to matter. The height of that peak as observed implies that there should be much less mass in the universe than is implied by observed gravity. Hence, there must be more mass, but the mass must not scatter CMB photons. But any baryonic matter must scatter CMB photons. So, the missing mass cannot be baryonic.

As somebody else already pointed out, you can come to a similar conclusion, based on gravitational lensing.

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The observational evidence for (cold, non-baryonic) dark matter can be grouped under the following headings (in the order I propose to address them):

- rich clusters (radial velocity dispersion of member galaxies, the Sunyaez-Zel'dovich effect, (strong) gravitational lensing, X-ray observations)

- galaxy halos (weak lensing, derived orbits of luminous objects in or just outside)

- (spiral) galaxy rotation curves

- CDM in, and near, the Milky Way (really a subset of galaxy rotation curves, but because it's so local, I'll treat it separately)

- poor clusters, galaxy groups (radial velocity dispersions, derived orbits of luminous objects in or near)

- large-scale structure and the CMB (observations provide indirect support for CDM).

If I have missed significant set of observations, that provide direct evidence of CDM, please add.

I don't think of it as significant, but I've seen a paper or two that attribute several observations to neutralino self-annihilation. These don't demand dark matter, and are still very questionable, but they include:
- excess heat emitted by the gas giants
- positrons coming from the center of the galaxy

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There is also the related issue of observations that suggest the dark matter cannot be baryonic. An example of that is observations of the outputs of cosmic nucleosynthesis, the light element ratios.