Hiding in the dark Constraining dark sectors with cosmological observations

Understanding the nature of dark matter (DM) remains one of the biggest open questions in fundamental physics. This thesis explores how we can utilize cosmological observations, such as the DM relic density, cosmic microwave background and big bang nucleosynthesis (BBN) observations to address this question. In particular, non-minimal models were studied in which dark matter lives in an extended dark sector containing additional particles beyond the DM candidate itself. These additional states can influence the resulting dark matter phenomenology.
Models that were studied include a pionic dark sector containing a resonant dark photon, in which the DM particle resembles a standard model pion, and an atomic DM setup, in which the DM is a bound state. The effect of resonantly enhanced annihilations, caused by the resonant dark photon on BBN was studied. Finally, significant emphasis was placed on dark matter (DM) production mechanisms, particularly on the generation of strongly interacting massive particle (SIMP) dark matter, whose relic abundance is determined by 3-to-2 self-interactions.