  We consider the vacuum energy of massive quantum fields in an expanding
universe. We define a conserved renormalized energy-momentum tensor by means of
a comoving cutoff regularization. Using exact solutions for de Sitter
space-time, we show that in a certain range of mass and renormalization scales
there is a contribution to the vacuum energy density that scales as
nonrelativistic matter and that such a contribution becomes dominant at late
times. By means of the WKB approximation, we find that these results can be
extended to arbitrary Robertson-Walker geometries. We study the range of
parameters in which the vacuum energy density would be compatible with current
limits on dark matter abundance. Finally, by calculating the vacuum energy in a
perturbed Robertson-Walker background, we obtain the speed of sound of density
perturbations and show that the vacuum energy density contrast can grow on
sub-Hubble scales as in standard cold dark matter scenarios.
