Effective description of dark matter as a viscous fluid
  Treating dark matter at large scales as an effectively viscous fluid provides an improved framework for the calculation of the density and velocity power spectra compared to the standard assumption of an ideal pressureless fluid. We discuss how this framework can be made concrete through an appropriate coarse-graining procedure. We also review results that demonstrate that it improves the convergence of cosmological perturbation theory. 
S. Floerchinger, M. Garny, N. Tetradis and U.A. Wiedemann
astro-ph.CO