  Results on particle creation from vacuum by the gravitational field of the
expanding Friedmann Universe are presented. Finite results for the density of
particles and the energy density for created particles are given for different
exact solutions for different regimes of the expansion of the Universe. The
results are obtained as for conformal as for nonconformal particles. The
hypothesis of the origination of visible matter from the decay of created from
vacuum superheavy particles identified with the dark matter is discussed.
