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The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itsel
by Florin_Andrei 10y ago
The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way.
That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter".
It's not a fluke in the equations, it's not a fifth fundamental force. All these alternatives have been shown to be very unlikely.
It's not "normal" matter because that one would interact in ways other than gravity, too, and it would tend to make "clumps" (stars, planets). This one doesn't make clumps.
So, yes, it's pretty certain that it exists, but nobody knows what it is. Its only attribute observed so far is its mass. Maybe we should call it "dark mass" instead.
- auvrw 10y agoupvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration? Edit: on second thought, a book reference is probably better
- mikeash 10y agoIn relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.
- effingwewt 10y agoThis is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.
- mikeash 10y agoSure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.
- nerfhammer 10y agoHow can it have mass / interact gravitationally and not make "clumps"?
- okket 10y agoSee http://physics.stackexchange.com/questions/214950/if-dark-matter-only-interacts-with-gravity-why-doesnt-it-all-clump-together-in http://physics.stackexchange.com/questions/214950/if-dark-ma... TL;DR: Dark Matter doesn't clump because of missing "friction" from the other forces, esp. electromagnetic/weak.
- Florin_Andrei 10y agoGravity only pulls things together, it doesn't stop them when they get close to each other. In the absence of "friction" (all other interactions except gravity), things get close to each other and just zoom past by / through each other. If you only have gravity but nothing else, there are no collisions. If you've seen the CUDA demo where they simulate thousands of bodies interacting by gravity, and they just keep swarming around forever - it's like that. No friction or stickiness, no collisions, just forever swarming around. Dark matter, whatever it really is, is rather like a "ghost". Just goes through normal matter and through itself and doesn't do anything except a little gravitational tug.