What is Dark Matter?

[pikachamp]

I've read several web pages about dark matter, and how it's a big deal among astrophysicists and such, but what is it? The descriptions are always vague, but I figure it's one of two things: Regular matter that's not being lit up by anything, and just sits there, invisible, or some weird exotic type of material that does weird stuff.

I'm pretty sure it's the former, but the descriptions are always so vague. Can anyone help me?

[Poisonium]

[worm]

the former is the basic idea. for us to detect stuff in space, we generally have to send or receive some form of electromagnetic radiation (light, radiowaves) and dark matter doesn't really emit or reflect enough for us to "see" it.

dark matter does affect visible matter around it and that's how we know it's out there.

[Courk]

I was going to come in here and make some sarcastic comment like "It's stuff that's not lit up." Except that would have been less sarcastic and more accidentally right. I never knew that that's what dark matter is, though I've never really pondered it.

[Samadhi]

[Chuck]

It seems obvious to me that there are three kinds of graviton, a strong attractor, a slightly weaker repulser, and a very weak repulser. The gravity we feel on earth is the difference between the attractor and the sum of the repulsers. The stronger repulser graviton is the least stable and decays over thousands of light years so the outer part of a galaxy is more influenced by the attractor and rotates more quickly. The attractor decays over millions of light years leaving the weak repulser to drive the acceleration of the expansion of the universe. There is no dark matter.

You can send my Nobel Prize by PayPal.

[Nsof]

I figured that since a) astrophysicists are looking for dark matter in space and that since b) space is cold, empty and dark place, i'll try to look for dark matter in my fridge.
Well, there is was; a dark somewhat disk-like mass which i can only (vaguely) describe as very hard, very cold and with no smell. I could not see its color because it was dark but then I opened the light and barbecued myself a no longer dark steak.
It was very good.
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Will sell this place for beer

[Matter*]

[Lepton*]

The "what" is hard to answer.... that's really the crux of the issue. The most likely candidate is an exotic heavy particle (such as we might soon discover in particle physics experiments). Some still cling to the idea that it might be planets or extremely dim stars (brown dwarfs). A final possibility is that the laws of physics are wrong, and that gravity works a bit different from how we expect.

There's the bell... I have a class to teach.
Stay in school and stay cool (as appropriate).

[Samadhi]

You forgot Dyson spheres and other megastructures.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Dented Ford]

[Samadhi]

[groza528]

I remember seeing a Dyson sphere on Star Trek once. For those unfamiliar, the idea is to completely enclose a star in order to use all of its energy. Being that it was only lit on the inside, it would appear dark to us.

[Coyote]

Forget Star Trek...we loyal Schlockers know the correct term for these little toys is buuthandi.

[ShadowSword]

[Courk]

Just 'cause he ate practically his entire planet doesn't mean he's obscure.

[Samadhi]

[MTGAP]

[Lepton*]

The astronomers are pretty sure that it's WIMPs (weakly-interacting massive particles), which means heavy particles. Examples:
1. The lightest supersymmetric particle in the "most obvious" supersymmetric extension of the standard model might be stable, but non-interacting, so that lots would be left over from the big bang.
2. Axions, which are required to make a certain elegant generalization of the math underlying the standard model.
3. There are lots of extra particles predicted by more advanced (and thus possibly more elegant) extensions of the standard model (ie: string theory, technicolor) that might be stable.
The particle physicists are holding their breaths until the LHC has had a couple years of data (and it's not even on yet! we're going to need a new generation of particle physicists who don't need oxygen soon) before they say what they are predicting. There are a few major camps, however:
1. The LHC finds nothing new. These are the cynical optimists that hope that the past 50 years of physics has completely overlooked a better explanation of the subatomic world. Naturally, this is a fringe group, but it is surprisingly popular.
2. The LHC finds the Higgs Boson (which explains the concept of "mass", which is an illusion but a heavy one) but nothing else. This is probably the majority opinion, and it's sort of depressing because, like with the first option, there will be no way to get funding for a bigger and better particle accelerator. There's also unanswered questions, because the standard model (with Higgs) is *essentially* complete, but has a few unanswered questions: dark matter, the matter/antimatter assymetry, and the reason that the universe's constants are what they are (physicists don't like the anthromorphic principle).
3. The LHC finds the Higgs Boson, plus evidence of a heavier particle. The heavier particle might be the dark matter. If this is the MSSM, then we might be done: just do a few more experiments to tie up loose ends (haha!). If not, there's a huge new theory to be explored and measured. This is the other most common viewpoint; it's pretty much where the foundation optimists sit.
4. The LHC finds something new. The real optimists are here, because that means lots of new questions, and lots of work to be done.

I hope that answers your questions.

[Lepton*]

PS: Pikachamp = Pika (as in pikachu, or as in the little furry animal) + Champ (as in, a winner)