Structure formation in a $\Lambda$ $viscous$ CDM universe
  The possibility of dark matter being a dissipative component represents an
option for the standard view where cold dark matter (CDM) particles behave on
large scales as an ideal fluid. By including a physical mechanism to the dark
matter description like viscosity we construct a more realistic model for the
universe. Also, the known small scale pathologies of the standard CDM model
either disappear or become less severe. We study clustering properties of a
$\Lambda$CDM-like model in which dark matter is described as a bulk viscous
fluid. The linear power spectrum, the nonlinear spherical "top hat" collapse
and the mass functions are presented. We use the analysis with such structure
formation tools in order to place an upper bound on the magnitude of the dark
matter's viscosity.
