Thermal Relic Density
After introducing the observed relic density of photons in Sect. 1.3 and the observed relic density of neutrinos in Sect. 2.1 we will now compute the relic density of a hypothetical massive, weakly interacting dark matter agent. As for the photons and neutrinos we assume dark matter to be created thermally, and the observed relic density to be determined by the freeze-out combined with the following expansion of the Universe. We will focus on masses of at least a few GeV, which guarantees that dark matter will be non-relativistic when it decouples from thermal equilibrium. At this point we do not have specific particles in mind, but in Chap. 4 we will illustrate this scenario with a set of particle physics models.