[raattgift]

> no apparent way to make slow-moving neutrinos

Adiabatic cooling of relic neutrinos (the cosmic neutrino background, CvB).

CvB formed before the cosmic microwave background and so is cooler than the CMB. Massless bosons like CMB photons have their wavelengths stretched through the history of the metric expansion of space; massive (even if the masses are small) fermions (like CvB neutrinos) instead see their speeds drop. The drop is about 161(1+z)/m with m being the neutrino mass and z the redshift; at present times CvB neutrinos are moving nonrelativistically (a couple thousand km/s) and so are certainly cold dark matter.

CvB neutrinos are incredibly abundant and do form a small fraction of CDM in the standard cosmology.

I don't think it's fair to call them WIMPs, though -- one wants invariant masses in the GeV range to gather up sufficient energy-density to drive galaxy cluster dynamics (cf. <https://en.wikipedia.org/wiki/Light_dark_matter>). CvB neutrinos are several orders of magnitude too light (invariant masses of 50-100 meV, kinetic masses of about 250 µeV).