  The radial orbit instability drives dark matter halos toward a universal
structure. This conclusion, first noted by Huss, Jain, and Steinmetz, is
explored in detail through a series of numerical experiments involving the
collapse of an isolated halo into the non-linear regime. The role played by the
radial orbit instability in generating the density profile, shape, and orbit
structure is carefully analyzed and, in all cases, the instability leads to
universality independent of initial conditions. New insights into the
underlying physics of the radial orbit instability are presented.
