IAAP,BNTRTOP but I assume you could count how many you detect to get a reasonable idea of how common they are in this part of the universe, at least. Then you could adjust it for the fact that WIMPs tend to prefer to hang out in gravity wells, like around here, and get a pretty good idea of the overall distribution. I haven't done the maths, but if you're detecting _any_ of 'em then they've got to be reasonably common, since they should be relatively uniformly distributed throughout the galaxy.

What I didn't understand until just now was this: if WIMPs interact by gravity why don't they fall into stars and planets, or even form stars and planets of their own? The answer is that they do fall into stars and planets, but just keep going through the middle (since they are not subject to any ordinary interactions) and out the other side. And they can't form dark-matter planets of their own since they don't interact strongly with each other either, so they won't stick together. Neato!