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.

Sorry. I know it, but every time I use the habitual notation, writing a post.

New Physical laws must be discovered. At least those ones, which explain why dark matter is stable.

Why do you think that it is not stable? If Dark Matter is unstable, why is there so much of it (25%) in the universe? Possibly it is and it has converted into Dark Energy in a manner analogous to E=MC^2?

But I wrote that it is stable. It can't be described by existing physical laws.
It is obvious that dark matter is stable; its particles don’t decompose (like electron, which is the lightest particle with electric charge and that is why it doesn’t separate into neutrino and photon, for example). Thus, there is undiscovered Law of Stability, which doesn’t allow these particle to decompose. Besides, particles of dark matter hardly interact with common matter.
By the way, there is a hypothesis that particles of dark matter are about 100-1000 heavier than proton and their interaction with common particles are similar to that with neutrino. This hypothesis claims that particles of dark matter born and annihilated intensively in the very early Universe at the temperature of 10E15 K (1000000000000000 K); and part of them lived out our days.

Your right. My fingers got ahead of my brain. So I'll show my flexibility by restating the question in reverse. Why do you think that Dark Matter is stable? Oh, you already answered that.

I'm not sure whether you are funning me or are seriously out of my league. My knowledge of current physics is sadly deficient. Hell, my formal study of the subject was limited to Newton, Maxwell, etc.-- mechanics/heat/sound, thermodynamics, electricity, optics and such. I graduated 15 years after Hiroshima and any advanced physics was largely Government Classified. My professional career was down-to-earth engineering (well, some occasional space-flight, but no further than the moon's surface).

Assuming that you are serious, "--an undiscovered Law of Stability???" "It is obvious that dark matter is stable; its particles don’t decompose." How can we know that?

I wonder if there is a book, "Modern Physics for Dummies".

If it wasn't stable, it would has already decomposed.



I'm not Physicist as well.



Don't forget that I'm Russian and all aricles I read about dark matter were in Russian. Maybe "Law of Conservation" is a more correct translation.



Really, unknown, undetectable matter... how can we know...
I tell about the hypothesis. Theoretical matter, theoretical properties and... calculations prove it, but... hypotetical dark matter itself is a result of wrong calculations (at least they weren't correct).
In short, we know nothing. It was a hypothesis, which explained why reality doesn't match with calculations. Scientists built a lot of colliders all over the world. They detect nothing except this one we discuss. Nobody doubts that flashes were really detected, but interpretations of these data can be very different.

Maybe it has a loooong half-life. We still have some naturally occuring radioactive materials on Earth, don't we? Hmm. Maybe that is because they were (and still are being) formed in stars that eventually blow up and spread their matter, some of which ends up here.

Using that logic, I would hypothesize that there is no dark matter analog to stars and nuclear reactions. If you mean our Law of Conservation of Mass + Energy, a similar law for the conservation of Dark Matter + Dark Energy would make sense; implying a constant for the Dark Matter to Dark Energy conversion (the Speed of Dark Light?) Quite true, but that one experiment does seem to be a hint that at least some of the hypotheses contain some degree of validity. Rather like Newton just after he first got the concept of mutual attraction of matter: we know a lot more about gravity today, but his work was seminal. I would expect that our knowledge of Dark Matter will snowball* now that we have a small start.

* American jargon for how a ball of snow increases in size as it rolls down a hill. It is probably in your dictionary as noun, but not as a verb.

How does the 'string' theory work into this?

Maybe. Maybe the products of their decomposition have a long period of half-life as well as the products of the decomposition of these products. Is there anything stable in this world?



Maybe similar, maybe not. Who knows? Especially about dark light...
Btw,this hypothesis assumes that Dark Matter is under antigravitation.



As for Newton... he used a concept of God to explain the stability of orbits. Now undiscovered force (matter and energy) was named Dark Matter and Dark Energy.

Yes. String theory -> supersymmetry -> dark matter consists of supersymmetrical partners of bosons, quarks and leptons.