Dissipation of dark matter
  Fluids often display dissipative properties. We explore dissipation in the
form of bulk viscosity in the cold dark matter fluid. We constrain this model
using current data from supernovae, baryon acoustic oscillations and the cosmic
microwave background. Considering the isotropic and homogeneous background
only, viscous dark matter is allowed to have a bulk viscosity $\lesssim 10^7$
Pa$\cdot$s, also consistent with the expected integrated Sachs-Wolfe effect
(which plagues some models with bulk viscosity). We further investigate the
small-scale formation of viscous dark matter halos, which turns out to place
significantly stronger constraints on the dark matter viscosity. The existence
of dwarf galaxies is guaranteed only for much smaller values of the dark matter
viscosity, $\lesssim 10^{-3}$ Pa$\cdot$s.
