Lights in a sea of darkness: constraining the nature and properties of dark matter using the stellar kinematics in the centres of ultra-faint dwarf galaxies

Dark matter is one of the biggest mysteries of the Universe. Its properties cannot be explained with the known laws of physics and elementary particles. Yet, it is the most abundant form of matter in the Universe.Several dark-matter theories exist, including cold dark matter (CDM), self-interacting dark matter (SIDM), and fuzzy dark matter (FDM). These theories make different predictions for the density profiles of dark-matter haloes (cuspy or cored), depending on the nature (CDM, SIDM, FDM) and properties (self-interaction strength, FDM particle mass) of dark matter. These profiles can be determined from the stellar kinematics of the galaxies hosted by the haloes. Many massive dwarf galaxies show cored profiles. However, baryonic processes such as star formation may also induce cores.In this thesis, I test CDM, SIDM, and FDM using the faintest and most dark matter–dominated galaxies, ultra-faint dwarf galaxies (UFDs), which offer a new perspective on the cusp–core problem. The stellar kinematics of UFDs should not be significantly affected by baryonic processes. I find that UFDs have no detectable cores, implying that the cores of more massive dwarf galaxies are not caused by dark-matter physics. I can also exclude the previously promising particle masses ~10^-22 eV/c^2 for FDM.Large scale structure and cosmolog