  Until now, there is no experimental evidence on the gravitational behaviour
of antimatter. While we may be confident that antimatter attracts antimatter,
we do not know anything on the interaction between matter and antimatter. We
investigate this issue on theoretical grounds. Starting from the CPT invariance
of physical laws, we transform matter into antimatter in the equations of both
electrodynamics and gravitation. In the former case, the result is the
well-known change of sign of the electric charge. In the latter, we find that
the gravitational interaction between matter and antimatter is a mutual
repulsion. This result supports cosmological models attempting to explain the
Universe accelerated expansion in terms of a matter-antimatter symmetry.
