  We consider, in general terms, the possible parameter space of thermal dark
matter candidates. We assume that the dark matter particle is fundamental and
was in thermal equilibrium in a hidden sector with a temperature $T'$, which
may differ from that of the Standard Model temperature, $T$. The candidates lie
in a region in the $T'/T$ vs. $m_{\rm dm}$ plane, which is bounded by both
model-independent theoretical considerations and observational constraints. The
former consists of limits from dark matter candidates that decoupled when
relativistic (the relativistic floor) and from those that decoupled when
non-relativistic with the largest annihilation cross section allowed by
unitarity (the unitarity wall), while the latter concerns big bang
nucleosynthesis ($N_{\rm eff}$ ceiling) and free streaming. We present three
simplified dark matter scenarios, demonstrating concretely how each fits into
the domain.
