Understanding the formation and evolution of non-linear structure in the universe can shed light on some of the most fundamental questions in Cosmology. In this talk I will talk about how the distribution of matter in dark matter halos can help us understand the nature of gravity, dark matter and the evolution of galaxies. In particular, I will focus on the splashback radius, a natural definition for the halo boundary. The splashback radius has emerged as probe of physics in the interface of galaxy formation and Cosmology. I will talk about recent developments, both from theoretical, and observational perspectives, and describe how current and future surveys will give us an exquisite view of the dynamics and evolution of dark matter halo.