  We review the hydrodynamics of the dark sector components in Cosmology. For
this purpose we use the approach of Newtonian gravitational instability, and
thereafter we add corrections to arrive to a full relativistic description. In
Cosmology and Astrophysics, it is usual to decompose the dark sector into two
species, dark matter and dark energy. We will use instead a unified approach by
describing a single unified dark fluid with very simple assumptions, namely the
dark fluid is barotropic and its sound speed vanishes.
