  We revive the historically first dark matter model based on neutrinos, but
with an additional assumption that neutrinos might exist in tachyonic almost
sterile states. To this end, we propose a group-theoretical algorithm for the
description of tachyons. The key point is that we employ a distinct tachyon
Lorentz group with new (superluminal) parametrization which does not lead to
violation of causality and unitarity. Our dark matter model represents
effectively scalar tachyonic neutrino-antineutrino conglomerate. Distributed
all over the universe, such fluid behaves as stable isothermal/stiff medium
which produces somewhat denser regions (`halos') around galaxies and clusters.
To avoid the central singularity inherent to the isothermal profile, we apply a
special smoothing algorithm which yields density distributions and rotation
curves consistent with observational data.
