  Dark matter may be coupled to dark radiation: light degrees of freedom that
mediate forces between dark sector particles. Cosmological constraints favor
dark radiation that is colder than Standard Model radiation. In models with
fixed couplings between dark matter and the Standard Model, these constraints
can be difficult to satisfy if thermal equilibrium is assumed in the early
universe. We construct a model of asymmetric reheating of the visible and dark
sectors from late decays of a long-lived particle (for instance, a modulus). We
show, as a proof-of-principle, that such a model can populate a sufficiently
cold dark sector while also generating baryon and dark matter asymmetries
through the out of equilibrium decay. We frame much of our discussion in terms
of the scenario of dissipative dark matter, as in the Double-Disk Dark Matter
scenario. However, our results may also be of interest for other scenarios like
the Twin Higgs model that are in danger of overproducing dark radiation due to
nonnegligible dark-visible couplings.
