  We model the dark sector of the cosmic substratum by a viscous fluid with an
equation of state $p=-\zeta \Theta$, where $\Theta$ is the fluid-expansion
scalar and $\zeta$ is the coefficient of bulk viscosity for which we assume a
dependence $\zeta \propto \rho^{\nu}$ on the energy density $\rho$. The
homogeneous and isotropic background dynamics coincides with that of a
generalized Chaplygin gas with equation of state $p = - A/\rho^{\alpha}$. The
perturbation dynamics of the viscous model, however, is intrinsically
non-adiabatic and qualitatively different from the Chaplygin-gas case. In
particular, it avoids short-scale instabilities and/or oscillations which
apparently have ruled out unified models of the Chaplygin-gas type. We
calculate the matter power spectrum and demonstrate that the non-adiabatic
model is compatible with the data from the 2dFGRS and the SDSS surveys. A
$\chi^{2}$-analysis shows, that for certain parameter combinations the
viscous-dark-fluid (VDF) model is well competitive with the $\Lambda$CDM model.
These results indicate that \textit{non-adiabatic} unified models can be seen
as potential contenders for a General-Relativity-based description of the
cosmic substratum.
