thanks for that really informative post, would you mind explaining what those eight observations are? I was only aware of the rate of expansion issues.
Off the top of my head, there’s rotation curves of galaxies, galaxy dynamics in clusters, gravitational lensing by galaxy clusters, and the frequency structure of the cosmic microwave background radiation. These are completely different kinds of observations, yet the values they give for the dark matter are all consistent. Then there are also particle physics models predicting various hitherto-unknown particles that could account for some or all of the dark matter, but those are pretty speculative.
Now, the dark energy, that’s just a label we give to a phenomenon we understand very little. We know something about the behavior it causes, but we don’t know what the cause itself is. It is quite possible to re-cast the phenomenon as a slight change to the laws of gravity, rather than as a new sort of energy, and in fact that’s the way it was originally described by Einstein. It’s really just a matter of aesthetics that it’s currently described as “energy” rather than as a modification to the laws of gravity.
Come now, that site is *clearly *a hoax. No woman that hot could be a physics geek… 
Chronos has nailed some of them; I will stick to the ones that are most compelling for me.
1.) Big Bang Nucleosynthesis, or the story of how the light elements were formed in the Universe. The amount of Deuterium, Helium-3, and Helium-4 we find in young gas clouds is determined by only one parameter: the ratio of normal matter to photons. Direct determination of this number tells us how much normal matter there is, and doesn’t let us have any more.
2.) Measurement of the speeds of galaxies in galaxy clusters tells us how much matter there needs to be in order to keep them gravitationally bound together. It’s about 6 times as much as we have in normal matter.
3.) Measurement of gravitational lensing (bending of light due to gravity) caused by galaxies and clusters tells us, independently, how much total matter is in the lensing objects. Again, about 5 to 6 times as much as we measure in normal matter.
4.) Large scale structure measurements: these are very sensitive to how much of the matter is normal (baryonic), and how much is “dark”. This is, in my opinion, the toughest obstacle for models with no dark matter, and one that no model has even attempted to explain. We quantify how this matter clusters by constructing a power spectrum, where we basically measure clustering as a function of scale. 100% Dark matter produces a perfectly smooth curve, while 100% normal matter produces a curve with strong forbidden zones. (I.e., the power drops to zero periodically.) What we observe is an almost perfectly smooth curve with small wiggles in it, with relative minima corresponding to the “zero points” in the 100% normal matter models. This is consistent with about 15% of the matter being normal matter and the remaining 85% being dark.
5.) Looking directly at a galaxy or cluster of galaxies that emits X-rays, we can measure that about 2% of the mass is in stars and about 11-13% of the mass is in gas and dust. The remaining? No normal signatures, except gravitational.
6.) The cosmic microwave background, another incredibly strong one. Tells us what the ratio of dark matter to normal matter is, with very little margin of error.
7.) The lack of MACHOs, a direct limit placed on collapsed baryonic structure in our galaxy. More evidence that not all of the matter in the galaxy can be accounted for with normal matter.
I’ve also written a little bit about it for the public in the past, which you’re free to browse through if you have the time and inclination: http://scienceblogs.com/startswithabang/dark-matter/
Wow, thanks a lot.
That’s a fair bit of reading for me.
But that leaves me with dozens of new questions. Particularly the “cannot collide” characteristic. Pretty much no two things can be in the same exact place, up until this. Doesn’t that characteristic imply a whole lot of things about dark matter?
All that orbital mechanics that leaves brown dwarves wandering in intergalactic space would seem to be more significant for dark matter particles, for instance. I mean, anything short of a black hole ends up being perfectly permeable to dark matter particles, right?
Wierd.
Tris
Not really. It just tells you that their cross-section is very, very small, but it doesn’t tell you whether it’s zero or just a tiny bit smaller than you’d be able to measure. But it tells you that if it does have an interaction with anything other than pure gravity, it’s much weaker than even the neutrino.
Dark matter is subject to gravitational ejection from 3-body interactions, same as brown dwarfs. And, as far as we’ve been able to tell thus far, yes, dark matter should be completely permeable even to a neutron star.
Heck, even neutrinos can pass pretty much unimpeded through a neutron star (the mean free path of a neutrino through a neutron star would be on the order of a thousand times greater than the star’s diameter). For any particle less reactive than neutrinos, all the more so.
It should be emphasized, by the way, that dark matter isn’t all one thing. A few percent of the dark matter in the Universe is perfectly ordinary baryonic matter that just happens to not be glowing: In fact, we’re made of dark matter. And of the non-baryonic dark matter (the weird stuff), neutrinos are at least some portion (though, it looks like, a very small one). For the rest of it, we don’t know what it is, and it might be all one kind of particle, but there’s really no reason to suspect that: Given how many different kinds of particles make up “ordinary” matter, it’s hardly unreasonable to suppose there are multiple dark matter particles, too.
Hopefully this won’t confuse anyone too much . . . although “dark matter” is used to mean “the matter that we can’t see” (i.e. not stars and such), people also sometimes use the phrase as shorthand for “non-baryonic dark matter”, particularly when speaking to the general public who doesn’t know what “baryonic” means anyway.
For instance most of the measurements ethansiegel mentions above are distinguishing normal baryonic matter from non-baryonic dark matter.
Baryons, incidentally, are just particles made of three quarks, such as protons and neutrons. There are other “normal” particles that aren’t baryons, such as electrons, but these are much less massive than protons and neutrons. Most of the mass of the universe from “normal” matter is protons and neutrons and nuclei made of protons and neutrons, hence baryonic.
OK here is my theory which is likely total garbage, but I’m a sensitive soul so take it easy on me when you shred it. What if there were an anti-gravitrons that are repelled by matter. They would congregate between the galaxies and “sqeeze” the ordinary matter in the galaxies. So there would be two forces holding galaxies together, gravity pulling everything towards the center and anti-gravity squishing everything from outside. I know there are probably a dozen reasons why this theory is wrong, but it allows the possibility of an anti-gravity hover-car, so I’m holding onto it.