Astrophysical data show that the major part of the matter in the Universe consists of “dark matter”. It was named ‘dark” because it is invisible in any diapason (X-ray, optical, ultraviolet etc). It reveals itself only by gravity influence on stars, galaxies and gas. The nature of the “dark matter” is also unknown. Astrophysicists suppose that the main components of it are WIMP — weakly interacting massive particles. They form huge clouds around every galaxy. Thus Solar system moving through WIMPs must be under influence of “WIMP wind”. At the same time the Earth goes round the Sun. So the Earth must experience changeable “head WIMP wind” with a one-year period.
WIMPs weakly interact with common matter, thus they easily come through the Earth. But sometimes they collide with nucleus, give them part of own impulse and knock out electrons. This process causes flares of light. So, it is possible to test the theory by measuring the frequency of such flares.


And it is finally detected.

“The highly radiopure $\simeq$ 250 kg NaI(Tl) DAMA/LIBRA set-up is running at the Gran Sasso National Laboratory of the I.N.F.N.. In this paper the first result obtained by exploiting the model independent annual modulation signature for Dark Matter (DM) particles is presented. It refers to an exposure of 0.53 ton$\times$yr. The collected DAMA/LIBRA data satisfy all the many peculiarities of the DM annual modulation signature. Neither systematic effects nor side reactions can account for the observed modulation amplitude and contemporaneously satisfy all the several requirements of this DM signature. Thus, the presence of Dark Matter particles in the galactic halo is supported also by DAMA/LIBRA and, considering the former DAMA/NaI and the present DAMA/LIBRA data all together (total exposure 0.82 ton$\times$yr), the presence of Dark Matter particles in the galactic halo is supported at 8.2 $\sigma$ C.L..”
. http://arxiv.org/abs/0804.2741v1

Interesting, but I want to wait until the discovery is independently verified. Of course, there is still the unresolved issue of Dark Energy, which takes up more of the universe than Dark Matter and ordinary matter combined.

After skimming the report I would say that it establishes a gold-standard for identifying and compensating for possible systematic effects and side-reactions. An 8.5 sigma confidence level is pretty convincing and--just visually--the processed data supports that level. Peer review comments seem to have been sparse and adequately addressed. All that is lacking is validation using a completely different experiment, although this experiment was repeated over several cycles with each cycle yielding consistent results.

Dark Matter! That used to be the stuff of Science Fiction. Now I would place it as highly likely on my belief scale.

Many physicists think that it is necessary to build the similar set-up to prove this theory. The new set up should be located in the Southern hemisphere and detectors mustn’t be of NaI(TI), but of KI or LiI.

. http://arxiv.org/PS_cache/arxiv/pdf/0804/0804.2741v1.pdf (the description of the DAMA/NaI).

This article is about debates: “Dark matter claims disputed”
. http://physicsworld.com/cws/article/news/33870

I can, in no way, match your scientific knowledge, Elena; however, isn't it true that science must first try to dis-prove itself before it can 'prove' itself? Science establishes a theory based on observed data. The only way it can establish that theory is by repeated attempts to establish other theories, through experimentation, until just one theory comes out on top. That can take years.

Why is dark matter important? Does it hold the universe together? I'd like to learn.

Dark matter is needed to plug some holes in General Relativity. General Relativity works well at predicting the motions of bodies in a system up to the level of a solar system, but falls flat on it's face when we try to apply it at the scale of a galaxy or cluster.

According to GR, galaxies should behave much like our solar system. As you move out from the center of the solar system, the circular velocity of a body in orbit around the sun begins to fall off. This doesn't happen in a galaxy though. Instead, near the outer edges the rotation curve simply becomes flat and doesn't change. If this happened in our solar system, then Pluto would have flown out of orbit and off into space somewhere, because it would have been moving to fast to stay put. The question is, why don't the stars at the outer edge of a galaxy fly off into empty space since they're not slowing down?

Well, some scientists came up with the idea of matter that you can't see(or even directly sense) to fill these holes. If you pile up all this invisible dark matter around the outside edges of a galaxy, it adds mass, and makes the galaxy much broader across than it appears, and thus explains why the rotation isn't slowing down. In other words, the outside edges aren't slowing down because they're not really the outside edges.

Of course, all this is well and good assuming General Relativity is correct and accurate at all scales. However, we've already seen GR fall apart at the quantum level, and I think it's certainly possible that it can fall apart at other levels of cosmology as well. If this whole dark matter thing turns out to be wrong, then it will probably take a new theoretical breakthrough to get their mathematical formulae back in line and working again.

Do I have to reconsider my meaning when I say, "You can stick it where the sun don't shine."?

I’ll tell about it in detail later. Briefly, scientists can’t make both ends meet in their calculations.
The calculations are correct only if our Universe has such structure: 5% - common matter, 0, 5% - stars, 0, 3-3% - neutrino, 25% - dark matter, 65-70% - dark energy. Of course, “dark matter” and “dark energy” are hypothetical substances. Physicists even say that the nature of dark energy is the main puzzle of fundamental Physics of XXI century.

Thank you for your explanation, Joblo, I kind of knew dark matter had something to do with holding the universe together, but couldn't remember the details. I also knew that GR doesn't work at the quantum level.

It is, however, a theory that has led to many advances in physics theory, which may lead to even more discoveries. That's what makes it all so exciting.

For those of you that may be baffled by Elena’s post: Much of the world uses commas to denote decimal points, whereas the USA uses periods. Translated into American, the above would be: 5% common matter, 0.5% stars, 0.3 to 3% neutrinos, 25% dark matter, and 65 to 70% dark energy.

As if that isn’t confusing enough, in the USA we would say that a railroad can climb a 3% grade, while elsewhere it would be a 30% grade. (In Japan for sure. Probably other countries as well.)

Of course we are right and they are wrong.