  A solution to the problem of the origin of matter in the universe can be
reasonably searched within extensions of the standard model that also explain
neutrino masses and mixing. Models embedding the minimal seesaw mechanism can
explain the observed matter-antimatter asymmetry of the universe via
leptogenesis and dark matter via active-sterile neutrino mixing. In this case a
keV lightest seesaw neutrino would play the role of warm dark matter particle.
This traditional solution is now constrained by various cosmological
observations. I will discuss the possibility that a much heavier but yet
metastable (dark) right-handed neutrino with mass in the $1\,{\rm TeV}$--$1 \,
{\rm PeV}$ range can play the role of (cold) dark matter particle. The right
abundance would be produced by the Higgs induced mixing with a seesaw
right-handed neutrino (RHINO model), i.e., by sterile-sterile neutrino mixing.
Such a mixing would necessarily require a further extension of the minimal
seesaw mechanism and can be described by a dimension-five effective operator.
The same mixing would also necessarily induce dark neutrino instability with
lifetimes that can be much longer than the age of the universe and can escape
current constraints from neutrino telescopes. On the other hand, a contribution
to very high energy neutrino flux produced by dark neutrino decays could
explain an anomalous excess at 100 TeV energies confirmed recently by the
IceCube collaboration. I will also discuss a simple UV completion where the
mediator is given by a massive fermion. Intriguingly, it comes out that the
favoured scale of new physics for RHINO to satisfy the dark matter requirements
coincides with the grand-unified scale: a RHINO miracle.
