  The cosmological observations suggest that the presently accelerating
universe should be filled by an exotic form of matter, violating the strong
energy condition, of unknown nature and origin. We propose the viscous dark
matter of a source of acceleration in the form of Chaplygin gas which is
characterized by equation of state in the phenomenological form
$p=-\frac{A}{\rho^{\alpha}}$, where $p$ and $\rho$ are pressure and energy
density respectively ($A$ and $\alpha$ are constants). Chaplygin gas is
interpreted in terms of viscous matter and without the cosmological constant.
The acceleration effect is caused only by viscosity in this class of
cosmological models. We show that bulk viscosity effects introduced to the
standard FRW cosmology give rise to the natural unification of both dark matter
and dark energy. We show that dust viscous cosmological models are structurally
stable if $m < 1/2$ ($1+\alpha=1/2-m$).
