  This paper aims to investigate the influence of inhomogeneous radiation in
void formation. Since the process of void formation is non-linear, a fully
relativistic model, which simulates the evolution of voids from the moment of
the last scattering until the present instant, is presented. It is found that
in order to obtain a model of a void which evolves from $\delta_0 = - 10^{-5}$
at the last scattering moment to the present day $\delta = - 0.94$, the
existence of radiation must be taken into account. The ratio of radiation
energy density to matter energy density in CDM models at the moment of last
scattering is 1/5. This paper proves that such a value of radiation energy
density cannot be neglected and influences the first stages of void evolution.
Namely, it is important to the process of structure formation and hence
significantly influences the dynamics of the Universe in the first millions of
years after the last scattering.
  From the fact that the evolution of voids proceeds differently in various
cosmological background models we use the process of void formation to put some
limits on values of cosmological parameters. We find that the model with
$\Omega_{mat} \approx 0.3$ fits the observational data best.
