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The Dark Matter Puzzle
We know dark matter is cold, but that's a new thing. We didn't back in my day when I started out, hot dark matter and warm dark matter were both very popular. The kinds of interactions that have been imagined for dark matter particles that would be of astrophysical interest have mostly been elastic collisions. So again, on the basis of data, we have excellent reason to believe the dark matter iscold, as well as dark.