Astronomical observations indicate an accelerated cosmic expansion, the cause of which is explained by the action of `dark energy'. Here we show that in discrete expanding space-time, only a tiny fraction of the vacuum fluctuations can become gravitationally effective and act as a driving `dark' agent. The analytically derived effective vacuum energy density is found to be closely related to the critical cosmic energy density, thus helping to solve the cosmological constant problem as well as the coincidence problem. The proposed model implies that in the present day universe only the vacuum field of the photon and that of the lightest neutrino contribute to the effective vacuum. This allows one to fix the neutrino masses within a narrow range. The model also implies that the (real) universe has to be considered as a thermodynamically open system which exchanges energy and momentum with the virtual reservoir of the vacuum.