  We investigate in detail the possibility that a single imperfect fluid with
bulk viscosity can replace the need for separate dark matter and dark energy in
cosmological models. With suitable choices of model parameters, we show that
the background cosmology in this model can mimic that of a LCDM Universe to
high precision. However, as the cosmic expansion goes through the
decelerating-accelerating transition, the density perturbations in this fluid
are rapidly damped out. We show that,although this does not significantly
affect structure formation in baryonic matter, it makes the gravitational
potential decay rapidly at late times, leading to modifications in predictions
of cosmological observables such as the CMB power spectrum and weak lensing.
This model of unified dark matter is thus difficult to reconcile with
astronomical observations. We also clarify the differences with respect to
other unified dark matter models where the fluid is barotropic, i.e., p=p(rho),
such as the (generalized) Chaplygin gas model, and point out their
observational difficulties. We also summarize the status of dark sector models
with no new dynamical degrees of freedom introduced and discuss the problems
with them.
