Are neutrinos definitely ruled out as Dark Matter?

[Orodruin]

This almost always adds new neutrino-like particles.

I think "almost always" is too strong to use here. There is no actual need to introduce new standard model singlet fermions. You can just as well add an SU(2) triplet scalar that effectively takes a small vev. This is the type-II seesaw mechanism. Essentially, what new states you need to add would typically depend on how you open the Weinberg operator at higher energies.

 

[nikkkom]

I think "almost always" is too strong to use here. There is no actual need to introduce new standard model singlet fermions.

Yes. That's why I used "almost always" phrase, not "always".