Dark matter caustics
  The late infall of cold dark matter onto an isolated galaxy such as our own produces flows with definite velocity vectors at any physical point in the galactic halo. It also produces caustics which are places where the dark matter density is very large. The outer caustics are topological spheres whereas the inner caustics are rings. The self-similar model of galactic halo formation predicts that the caustic ring radii $a_n$ follow the approximate law $a_n \sim 1/n$. In a recent study of 32 extended and well-measured galactic rotation curves, we found evidence for this law. 
P. Sikivie and W. Kinney (U of Florida)