Xiaolong Du is a postdoctoral associate at the Carnegie Observatories. His work focuses on cosmological structure formation in different dark matter models and on constraints on these models from gravitational effects such as substructure lensing. Using idealized numerical simulations and semi-analytic models, he tries to find an accurate, but fast, way to study the behaviors of dark matter on sub-galactic scales, where different dark matter candidates may behave very differently.