| Abstract: The hypothesis of Cold Dark Matter (CDM) has been spectacularly confirmed on the largest scales of the Universe and must now be stress-tested on sub-galactic scales. Many well-motivated and generic alternatives to CDM can leave spectacular signatures on precisely these scales, affecting the evolution of galaxies as well as their population statistics. As a concrete example, I will present the results of the first cosmological hydrodynamical simulations where the majority of the dark matter is collisionless, but a sub-component is strongly dissipative minimal Atomic Dark Matter. I will discuss the impact of the dissipative dark matter on both the formation of a Milky Way-like galaxy, as well as on its dwarf galaxies and dark substructure. The results demonstrate that galaxies are extremely sensitive probes for dark-sector physics---even a small fraction of dark matter with non-trivial dynamics can, in some cases, be enough to destroy a galaxy. Link to the Event Video |