This idea struck me, and I don't know if it has any merit, or if it does, whether someone else has already considered it, but....
Maybe dark matter is just "normal" matter existing in the curled up extra dimensions predicted by string theory, superstring theory, and M-theory. Seems like a good place to hide most of the Universe if you ask me.
UPDATE: I did a quick Google search before I posted this and came up with no matches, but later a more in-depth search shows that others have had this idea before me. Good. Now I hope someone figures out a good way to test it.
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