Tachyonic approach to neutrino dark matter
  We apply a new approach based on three relativistic groups (bradyon, tachyon
and instanton) forming the `Lorentz groupoid' which allows, in particular, to
consider tachyons without introducing imaginary masses and negative energies
(related, as known, to violation of causality and unitarity). This leads to
effectively scalar conglomerate composed of tachyonic neutrino and antineutrino
spinor wave functions as a viable model for stationary dark matter. We also
briefly discuss a relevant early non-stationary high-energy stage of the
universe evolution.
