  In an expanding universe the vacuum energy density \rho_{\Lambda} is expected
to be a dynamical quantity. In quantum field theory in curved space-time
\rho_{\Lambda} should exhibit a slow evolution, determined by the expansion
rate of the universe H. Recent measurements on the time variation of the fine
structure constant and of the proton-electron mass ratio suggest that basic
quantities of the Standard Model, such as the QCD scale parameter
\Lambda_{QCD}, may not be conserved in the course of the cosmological
evolution. The masses of the nucleons m_N and of the atomic nuclei would also
be affected. Matter is not conserved in such a universe. These measurements can
be interpreted as a leakage of matter into vacuum or vice versa. We point out
that the amount of leakage necessary to explain the measured value of
\dot{m}_N/m_N could be of the same order of magnitude as the observationally
allowed value of \dot{\rho}_{\Lambda}/\rho_{\Lambda}, with a possible
contribution from the dark matter particles. The dark energy in our universe
could be the dynamical vacuum energy in interaction with ordinary baryonic
matter as well as with dark matter.
