  We investigate in massive early-type galaxies the variation of their
two-dimensional central fraction of dark over total mass and dark matter
density as a function of stellar mass, central stellar velocity dispersion,
effective radius, and central surface stellar mass density. We use a sample of
~ 1.7 x 10^5 galaxies from the SDSS DR7 at redshift smaller than 0.33. We apply
conservative photometric and spectroscopic cuts to select galaxies with
physical properties similar to those of the lenses studied in the SLACS Survey.
The values of the galaxy stellar and total mass projected inside a cylinder of
radius equal to the effective radius are obtained, respectively, by fitting the
SDSS multicolor photometry with stellar population synthesis models, under the
assumption of a Chabrier stellar IMF, and adopting a one-component isothermal
total mass model with effective velocity dispersion approximated by the central
stellar velocity dispersion. We find that within the effective radius the
stellar mass estimates differ from the total ones by only a relatively constant
proportionality factor. In detail, we observe that the values of the projected
fraction of dark over total mass and the logarithmic values of the central
surface dark matter density (measured in M_{Sun} kpc^{-2}) have almost Gaussian
probability distribution functions, with median values of 0.64^{+0.08}_{-0.11}
and 9.1^{+0.2}_{-0.2}, respectively. Our results disfavor an interpretation of
the tilt of the FP in terms of differences in the galaxy dark matter content
and give useful information on the possible variations of the galaxy stellar
IMF and dark matter density profile. Finally, we provide some observational
evidence on the likely significant contribution of dry minor mergers, feedback
from active galactic nuclei, and/or coalescence of binary black holes on the
formation and evolution of massive early-type galaxies.
