  In the last years, we saw more and more attempts to explain dark matter as a
general relativistic effect, at least for some fraction. Following this
philosophy, we considered the gravitational distortions due to the
inhomogeneous distribution of matter in the universe, which we know from
general relativity to be retarded distortions. This provides a magnification
effect, since the distortions we feel now depend not on the present matter
density, but on the past one, which is greater. The expansion rate of the
universe is perturbed as well, in a not negligible way, despite matter
inhomogeneities are small, because of the same magnification effect. The
deceleration parameter, which is a way to evaluate the quantity of dark matter
in the universe, is perturbed in turn, so that the real amount of dark matter
is less than what is usually believed.
