The domain of thermal dark matter candidates

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.