Abstract: The canonical Lambda CDM model predicts the existence of light dark matter subhalos, invisible to traditional luminous tracers. In contrast, many theoretically well motivated particle dark matter candidates modify the matter power spectrum or provide channels for evolution of subhalo structure, leading to distinct predictions from Lambda CDM. In this work I provide an overview of dark matter substructure in Lambda CDM as well as some of it alternatives and I discuss the exciting prospects to test theories of dark matter against the large amount of upcoming data from The Roman Space Telescope and LSST. I then go into more depth in using stellar streams as tracers of invisible dark matter halos as well as the predictions of self-interacting dark matter in galaxy-galaxy lensing systems.