Astrophysical Probes of the Dark Sector in the Nightmare Scenario

The particle identity of the dark sector is poorly understood and constraints on its possible non-gravitational interactions with the Standard Model have become increasingly stringent over the last decade. With the lack of convincing experimental evidence for such interactions, the possibility that its particle nature can only be probed via gravitational interactions (the "Nightmare Scenario") must be addressed. Galactic and sub-galactic astrophysical structure provides crucial probes into the microscopic physics of the dark sector and its dark matter content. For example, dark matter with sizable self interactions, dissipation, or internal degrees of freedom, can leave distinctive signatures on the properties of galaxies and their satellites. I will discuss how to probe the parameter space of such theories using galactic substructure and show how dark sector microphysics can potentially be discovered through such gravitational probes.