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It seems widely stated, but not thoroughly explained, that Dark Matter is not normal matter as we understand it. Wikipedia states "Consistency with other observations indicates that the vast majority of dark matter in the universe cannot be baryons, and is thus not formed out of atoms."

How can we presume to know this? Our best evidence for such dark matter is the rotational speeds of galaxies. It sounds like we can measure/approximate the gas density and stellar masses somehow, yet I don't understand how we can account for things like planets, asteroids, black holes without accretion disks, and other things that have mass but don't glow. How is it we dismiss these explanations for it, and jump right to WIMPs and other exotic explanations?

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You want to search on "Micro-lensing" and "MACHOs" in conjunction. The density of compact, cold baryonic object in middle masses is well measured for the Milky Way. It is far below that needed to account for the rotation curve. –  dmckee Apr 5 '12 at 19:45
Possible duplicate: physics.stackexchange.com/q/1008/2451 –  Qmechanic Feb 2 '13 at 11:57
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up vote 11 down vote accepted

Definitely see the comments on your question. But a very brief outline of the data:

Rotation-curves and galaxy-cluster mass measurements show the detailed distribution of matter in those objects, the amount of mass far exceeds the observed mass ---> most mass is non-observed

Gravitational-lensing searches show that the "dark-matter" constituents must be composed of objects less than about $10^{-7} \textrm{ M}_\odot \sim 0.03 \textrm{ M}_\oplus$, i.e. it must be asteroid size or smaller. Asteroid size can't really form stably (in such large amounts), and would be rapidly accreted by larger mass objects --> dark-matter constituents must be small.

Baryonic matter which is massive and small is constrained to gas and dust. Both of these things, when hot, are easily observable (especially in hot galaxy clusters)... yet the premise is that we can't see them --> dark matter is not baryonic

There is lots more evidence, this is just the most basic outline. The biggest additional piece overall is from cosmology: anisotropies in the cosmic microwave background tell you a lot about the initial universe and the seeds of structure formation -- comparing that with what we see in the current universe tells us about the evolution of structure in the universe, which ends up requiring that the dominant component of mass in the universe has no pressure which again rules out baryonic material. There's still more evidence.... but I'm not expert enough to try to explain it.

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Can you explain for a layman, does that mean that the masses of typical dark matter chunks are less than that of asteroids, or that the chunks of dark matter occupy about that much volume, so that there are asteroid-sized blobs of dark matter floating around everywhere? –  Rei Miyasaka Jan 2 '13 at 15:36
That's a good - and actually quite complicated - question. To my knowledge, most of the constraints on dark-matter are actually most accurately expressed in terms of density in some region (i.e. the universe as a whole, or the average over a galaxy-cluster, or the average-density at a certain distance from a galaxy center). You can make statements about mass of particles using statistical smoothness arguments, but volume isn't really constrained---I don't think. Even the concept of volume for non-baryonic matter is non-trivial to define / constrain. –  zhermes Jan 2 '13 at 20:43
Note that, the effects of DM depend on density (and sometimes mass-per-particle), but volume doesn't really matter---as long as its vaguely comparable or less than the separation between particles... –  zhermes Jan 2 '13 at 20:44
Ahh, got it -- more or less. Thanks! –  Rei Miyasaka Jan 2 '13 at 22:36
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CDM has not been found in any collider experiment in the last 40 years. What are the odds of something as massive as CDM and 5 times as dense as regular matter not being involved in one of these collisions?

As for the evidence that the CMB radiation tells us there's CDM, this is circular logic. The CMB is evaluated using Lambda-CDM assumptions, so it is no wonder that you get Lambda-CDM answers out of the application of the model to the data. There is also Occam's Razor which basically states that you can model just about anything if you throw enough parameters at it, which is basically what the 'standard' model does. Just because the Ptolemiacs could predict the retrograde of Mars doesn't mean they had a clue about how the solar system worked.

If CDM existed as particles, they would have been found by now. The fact is that the model is seriously broken but so many careers hang on publishing papers about magic particles and phantom energy is has become self-perpetuating business, just like astrology was 400 years ago (before Copernicus).

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"If CDM existed as particles, they would have been found by now." Not necessarily true, as even a basic grasp of weak scale physics will tell you. Direct detection experiments are only just beginning to eat into the interesting parameter space of the usual models. –  Michael Brown Nov 1 '13 at 2:39
If you have doubts about CDM (and we all do) it's far more constructive to propose a concrete alternative and try to constrain it. This is what has already been happening for years. If you actually read the literature it's not like nobody is talking about modified gravity or other such things. It's just not working as well as CDM (yet). –  Michael Brown Nov 1 '13 at 2:44
By this argument, would the Higgs have been just a "magic particle" until a few months or years ago (depending on your confidence in statistical data)? Doesn't the Higgs show that the process of mostly theoretical/indirect evidence leading to a concrete search and finally a discovery can work? –  Kevin Driscoll Dec 20 '13 at 21:47
@Kevin Driscoll - The Higgs particle was the last undiscovered particle predicted by the Standard Model. That is, it had a solid theoretical foundation before they found it. Dark Matter is not predicted. You need to resort to Super-symmetry in order to find an electrically neutral particle that doesn't decompose in a couple of minutes. Supersymmetry has all but been ruled out by the LHC experiments as the Higgs particle was found nearly exactly where the standard model predicted it would be. –  DRAirey1 Jan 3 at 0:26
@Michael Brown - OK, how is this then: Space is equal to the square of time. That simple relationship explains the dynamics of SNe Ia better than Lambda-CDM and doesn't require violating the First Law of Thermodynamics or any exotic particles to do it. –  DRAirey1 Jan 3 at 0:30
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