Right-handed neutrinos are a prime candidate for dark matter. The possibilities and exact contributions depend on whether neutrinos are massive or not.

The article glosses over the fact that massive neutrinos are not part of the mainstream Standard Model, but depend on semi-detached and unconfirmed extensions, such as Majorana spinors and (P)MNS mixing matrix:

https://en.wikipedia.org/wiki/Pontecorvo%E2%80%93Maki%E2%80%...

Maybe it does. Maybe it's the key to everything existing as we know it! But it sure would be interesting to understand why, if that is the case. The most reliable models we have for how the natural world works tend to be very simple in their mathematics, as long as we're using the right mathematics to model them.