[roywiggins]

Neutrinos are almost as strange. They interact a tiny bit via the weak force, and they interact via gravity like everything else, but that's it. We know they exist because of the weak interaction.

Maybe dark matter interacts with things via the weak force, but even less than neutrinos- not quite zero, but just small enough that we can't measure it, or can't measure it yet. Hopefully, they do interact a little bit by some other mechanism otherwise they'll be very hard to detect- I believe this is what current attempts to find dark matter are relying on.