The neutrino is perhaps the most elusive member of the particle zoo.
The questions about its nature, namely: Dirac or Majorana, the value
of its mass and the interactions with other particles, the number of its
components including sterile species, are long standing ones and still re-
main in large extent without conclusive answers. From the side of the
nuclear structure and nuclear reactions, both theories and experiments,
the need to elucidate these questions have, and still has, prompt crucial
developments in the fields of double beta decay, double charge exchange
and neutrino induced reactions. The measurements of neutrino flavor os-
cillation parameters contribute largely to restrict models with massless
neutrinos. From the particle physics side, the possibilities to extent the
standard model of electroweak interactions to incorporate a right handed
sector of the electroweak lagrangian are directly linked to the adopted neu-
trino model. Here, I would like to address another aspect of the problem
by asking the question of the neutrino mass mechanism in the cosmological
context, and particularly about dark matter.