  Dark sectors with purely gravitational couplings to the Standard Model are
unavoidably populated from the SM plasma by graviton exchange, and naturally
provide DM candidates. We examine the production in the relativistic regime
where the dark sector is approximately scale invariant, providing general
analytical formulas that depend solely on the central charge of the dark
sector. We then assess the relevance of interactions that can lead to a variety
of phenomena including thermalisation, nonperturbative mass gaps,
out-of-equilibrium phase transitions and cannibalism in the dark sector. As an
illustrative example we consider the dark glueball scenario in this light and
show it to be a viable DM candidate due to the suppression of gravitational
production. We go on to extend these results to strongly coupled CFTs and their
holographic duals at large-N with the dark dilaton as the DM candidate.
