regarding your note on the Neff=3 obtained from planck, I think this number only evaluates “radiant” neutrino species. ie. heavy neutrinos that travel slower than c would probably either not appear at all or weigh much less than 1. Than again im not a professional in neutrino physics. Still I feel that heavy sterile neutrinos are actually a good explanation for a CDM particle, requiring less “new physics” than other dark matter particles.

Are there observations that proove that dark matter interacts weakly thus justifying a WIMP ? I think not. Much rather is the absence of such a result a hint that dark matter is indeed purely gravitational.