"Dark matter" as I understand it is a slang term for the as yet unknown answer to the question "Why does everything really big move as if it were a lot more massive than we observe it to be?"
Planets and non-glowy stuff was a first suggestion, but doesn't account for nearly enough mass- you run in to problems where as the proposed density of cold stuff increases to fit the required mass into the required space, you start to get absorbtion and re-emission effects. (eg, the view of the (bright) center of our galaxy is visible-light dim from here due to dust in the way, but you get a big radio signal)
http://astron.berkeley.edu/~mwhite/darkmatter/dm.html has much better clue than I.
Theories as to what it actually is range from the complicated to the truly wacky, but that's normal for a new question. It takes a while to figure out what question you're asking, let alone test the answers that people come up with.
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"Dark matter" as I understand it is a slang term for the as yet unknown answer to the question "Why does everything really big move as if it were a lot more massive than we observe it to be?"
Planets and non-glowy stuff was a first suggestion, but doesn't account for nearly enough mass- you run in to problems where as the proposed density of cold stuff increases to fit the required mass into the required space, you start to get absorbtion and re-emission effects. (eg, the view of the (bright) center of our galaxy is visible-light dim from here due to dust in the way, but you get a big radio signal)
http://astron.berkeley.edu/~mwhite/darkmatter/dm.html has much better clue than I.
Theories as to what it actually is range from the complicated to the truly wacky, but that's normal for a new question. It takes a while to figure out what question you're asking, let alone test the answers that people come up with.