dark matter

[Johnny Pneumatic]

what is it?

 

[LFTKBS]

If I knew the answer to that, I'd be a lot more famous than I am now.

 

[geni]

what is it?

There are some questions that should not be asked, this is one of them (because the answer will make your brain disolve about half way through).

 

[Ziggurat]

Matter that accounts for the observed gravitational attraction of galaxies which can't be observed by light. If we look at other galaxies, we can calculate their mass by watching how fast they spin. The amount of mass they have is greater than we calculate by looking at how many stars are in the galaxy and how much visible dust is there, so there's some missing "dark" (as in, doesn't interact with light much) matter. What exactly that matter IS is still a mystery. The candidates fall into two basic camps:

MACHOs (Massive compact halo objects): things like brown dwarves, small stars that are a little too small to ignite, so they're close to invisible from far away. It's almost certain that such brown dwarves exist, but it's hard to see how there could be enough of them.

WIMPs (weakly interacting massive particles): maybe there are elementary particles out there with mass (don't actually have to be heavy compared to other particles, as long as they're not massless), which would interact with gravity according to their mass but might interact with the other fundamental forces VERY weakly, as neutrinos do (I don't think neutrinos can account for the missing mass, though). If such particles exist, detecting them directly might be close to impossible.

 

[WanderingKnight]

MACHOs (Massive compact halo objects):

Minor nitpick: MACHO: Massive, Astronomical, Compact Halo Objects.

Sort of a moot point, since it's a backronym. WIMP was the first term created, and someone whimsically suggested the alternative theory should be called MACHO's and then found an acronym that fit the letters.

 

[Johnny Pneumatic]

Gotta love acronyms. NASA, AMANDA, NATO, MASER, LASER,........

 

[wollery]

Minor nitpick: MACHO: Massive, Astronomical, Compact Halo Objects.

Sort of a moot point, since it's a backronym. WIMP was the first term created, and someone whimsically suggested the alternative theory should be called MACHO's and then found an acronym that fit the letters.

Minor nitpick, Ziggurat was correct, MAssive Compact Halo Objects.

But it's even more of a moot point since the evidence shows that although there are some MACHOs out there, there aren't nearly enough to account for the missing matter (MACHO collaboration, OGLE and EROS projects). This of course means that if there is dark matter it must be in the form of WIMPs.

Alternatively dark matter isn't the answer, if Mordehai Milgrom is right then our theories of gravitation need to be revised so that under low acceleration conditions (such as you find in the outer reaches of galaxies) the constant of gravitation isn't actually constant. This can account for most of the problems that dark matter is being invoked to solve.