The possibility that dark matter at large scales admits a description as a
viscous fluid is discussed. The effects of shear viscosity are examined in
two scenarios:
1) Dark matter with shear viscosity arising from its fundamental
interactions: The back-reaction of fluctuations on the average energy
density could accelerate the cosmological expansion.
2) Conventional dark matter in a coarse-grained description: The effect of
short-distance (galactic-scale) fluctuations can be mapped onto pressure
and viscosity terms in the effective theory of long-distance
perturbations. The two-loop nonlinear power spectrum of the long-distance
modes within the effective theory agrees well with N-body simulations and
displays better convergence properties than standard perturbation theory.