Abstract: Structure in the universe arises from the non-linear collapse of primordial perturbations, forming dark matter halos that host galaxies at their centers. Despite the complexity of this process, the resulting dark halos are thought to obey a number of universal laws. I connect simple, sometimes analytical modeling with supercomputer simulations to show that the structure of dark halos is intimately connected to their history and dynamics. Contrary to conventional wisdom, I demonstrate that halos have a well-defined physical boundary called the splashback radius. I explore this radius in detail by tracking billions of simulated particles, and discuss how its recent discovery in the real universe marks the beginning of a new era for observations of the halo outskirts.