Viscous dark matter

We discuss two aspects of the scenario in which dark matter possesses (fundamental or effective) viscosity: a) Large shear viscosity can induce the acceleration of the average expansion through the backreaction of fluctuations. b) Treating dark matter at large scales as an effectively viscous fluid provides an improved framework for the calculation of the matter power spectrum. We use intuition from the Wilsonian renormalization group in order to make this framework concrete, and review results that demonstrate that it improves the convergence of cosmological perturbation theory.