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Originally Posted by Shpoop
So, just curious here, what do you see? If light doesn't reflect off it, it isn't visible, but you cannot see through it, right? So then its just a big black blob, right?
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If it was black, it would be soaking up E-M radiation.
There is the 'dim body' theory to explain dark matter. Most of the theories I've heard actually result in matter that interacts very weakly electro magnetically.
This changes how it behaves a whole bunch. No radiating heat, cooling down, and collapsing into a star, then shining brightly -- instead, you get matter that forms huge clouds only held together by gravity, and the particles in it moving in huge lopsided orbits around and through the cloud (for example). The particles wouldn't even collide with each other (bright matter's collision is mostly E-M based: your electrons push off against each other due to their electric charge).
This allows there to be alot more of it in an area without it doing bright, and apparently absent, things like collapsing into stars or showing up by occulding light coming from behind it.
If that dark matter galaxy pans out, that is an example of how you would see 'truely dark' matter. E-M wise, you can only easily detect it by how it tugs 'bright' matter around (in this case, hydrogen gas).