  If, during the early Universe epoch, the dark matter particle thermalizes in
a hidden sector which does not thermalize with the Standard Model thermal bath,
its relativistic thermal decoupling can easily lead to the observed relic
density, even if the dark matter particle mass is many orders of magnitude
heavier than the usual $\sim$ eV hot relic mass scale. This straightforward
scenario simply requires that the temperature of the hidden sector thermal bath
is one to five orders of magnitude cooler than the temperature of the Standard
Model thermal bath. In this way the resulting relic density turns out to be
determined only by the dark matter mass scale and the ratio of the temperatures
of both sectors. In a model independent way we determine that this can work for
a dark matter mass all the way from $\sim 1$ keV to $\sim 30$ PeV. We also show
how this scenario works explicitly in the framework of two illustrative models.
One of them can lead to a PeV neutrino flux from dark matter decay of the order
of the one needed to account for the high energy neutrinos observed by IceCube.
