  We propose a scenario in which the dark components of the Universe are
manifestations of a single bulk viscous fluid. Using dynamical system methods,
a qualitative study of the homogeneous, isotropic background scenario is
performed in order to determine the phase space of all possible solutions. The
specific model which we investigate shares similarities with a generalized
Chaplygin gas in the background but is characterized by non-adiabatic pressure
perturbations. This model is tested against supernova type Ia and matter power
spectrum data. Different from other unified descriptions of dark matter and
dark energy, the matter power spectrum is well behaved, i.e., there are no
instabilities or oscillations on small perturbation scales. The model is
competitive in comparison with the currently most popular proposals for the
description of the cosmological dark sector.
