Abstract: Structure in the universe arises from the nonlinear 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.