  We show that hidden hot dark matter, hidden-sector dark matter with
interactions that decouple when it is relativistic, is a viable dark matter
candidate provided it has never been in thermal equilibrium with the particles
of the standard model. This hidden hot dark matter may reheat to a lower
temperature and number density than the visible Universe and thus account,
simply with its thermal abundance, for all the dark matter in the Universe
while evading the typical constraints on hot dark matter arising from structure
formation. We find masses ranging from ~3 keV to ~10 TeV. While never in
equilibrium with the standard model, this class of models may have unique
observational signatures in the matter power spectrum or via extra-weak
interactions with standard model particles.
