  We consider the Universe at the late stage of its evolution and deep inside
the cell of uniformity. At such scales, the Universe is highly inhomogeneous
and is filled with inhomogeneities in the form of galaxies and the groups of
galaxies. We also suggest that the Universe is filled with a perfect fluid, and
its fluctuations have peculiar velocities of the same (non-relativistic) order
of magnitude as for the inhomogeneities. In this sense, the inhomogeneities
(e.g. galaxies) and fluctuations of perfect fluids are coupled with each other.
We clarify some important points of this approach and present a brief review of
previous studies (e.g. CPL model and Chaplygin gas). We demonstrate that
considered perfect fluids which satisfy our approach are really coupled to
galaxies concentrating around them. The averaged (over the whole Universe)
value of their fluctuations is equal to zero.
