  If dark matter is light, it may be due to a seesaw mechanism just as
neutrinos are. It is postulated that both originate from the same type of heavy
fermion anchors, either singlets or triplets. In the latter case, a shift of
the $W$ mass is predicted, as suggested by the $CDF$ precision measurement. A
spontaneously broken dark $U(1)$ gauge symmetry is assumed, resulting in
freeze-in long-lived light dark matter.
