Hiding an elephant: heavy sterile neutrino with large mixing angle does
  not contradict cosmology
  We study a model of a keV-scale sterile neutrino with a relatively large
mixing with the Standard Model sector. Usual considerations predict active
generation of such particles in the early Universe, which leads to constraints
from the total Dark Matter density and absence of X-ray signal from sterile
neutrino decay. These bounds together may deem any attempt of creation of the
keV scale sterile neutrino in the laboratory unfeasible. We argue that for
models with a hidden sector coupled to the sterile neutrino these bounds can be
evaded, opening new perspectives for the direct studies at neutrino experiments
such as Troitsk $\nu$-mass and KATRIN. We estimate the generation of sterile
neutrinos in scenarios with the hidden sector dynamics keeping the sterile
neutrinos either massless or superheavy in the early Universe. In both cases
the generation by oscillations from active neutrinos in plasma is suppressed.
