> This requires introducing a right-handed partner for each known (left-handed) neutrino, which some people don't like because right-handed particles don't par

> This requires introducing a right-handed partner for each known (left-handed) neutrino, which some people don't like because right-handed particles don't partake in weak interactions, and weak interactions are the only (known) neutrino interactions (apart from gravity), so you end up with undetectable particles. For anyone else wondering: yes, this does make said right-handed (aka sterile) neutrinos a candidate for dark matter, assuming some of them are much heavier than their left-handed counter parts to account for the 'cold' properties of dark matter that we observe.