Warps and Cosmic Infall
  N-body simulations show that when infall reorientates the outer parts of a galactic halo by several degrees per Gyr, a self-gravitating disk that is embedded in the halo develops an integral-sign warp that is comparable in amplitude to observed warps. Studies of angular-momentum acquisition suggest that the required rate of halo reorientation is realistic for galaxies like the Milky Way. 
Ing-Guey Jiang and James Binney (Theoretical Physics, University of   Oxford)