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 .

PACS numbers : 98.80-k , 95.36+x