  We suggest that the eventual gravitational repulsion between matter and
antimatter may be a key for understanding of the nature of dark matter and dark
energy. If there is gravitational repulsion, virtual particle-antiparticle
pairs in the vacuum, may be considered as gravitational dipoles. We use a
simple toy model to reveal a first indication that the gravitational
polarization of such a vacuum, caused by baryonic matter in a Galaxy, may
produce the same effect as supposed existence of dark matter. In addition, we
argue that cancellation of gravitational charges in virtual
particle-antiparticle pairs, may be a basis for a solution of the cosmological
constant problem and identification of dark energy with vacuum energy. Hence,
it may be that dark matter and dark energy are not new, unknown forms of
matter-energy but an effect of complex interaction between quantum vacuum and
known baryonic matter.
