My work there focused on neutrino astrophysics and cosmology. During this time I also developed the ``Hidden Valley'' models which opened new directions to search for low mass hidden sectors at colliders such as the LHC. These classes of models also became central to my subsequent work in dark matter, including Asymmetric Dark Matter and hidden sector dark matter more generally. I became a postdoc at the University of Wisconsin, Madison and then the David Schramm Fellow at Fermi National Accelerator Laboratory. During this time, I focused on developing concrete models of hidden sector dark matter. I instigated theories of ``Asymmetric Dark Matter'' and established techniques to build models of natural low mass hidden dark matter sector.