  The dark matter content of early,- type galaxies (ETGs) is a hotly debated
topic with contrasting results arguing in favour or against the presence of
significant dark mass within the effective radius and the change with
luminosity and mass. In order to address this question, we investigate here the
global mass - to - light ratio $\Upsilon(r) = M(r)/L(r)$ of a sample of 21
lenses observed within the Sloan Lens ACS (SLACS) survey. We follow the usual
approach of modeling the galaxy as a two component systems, but we use a
phenomenological ansatz for $\Upsilon(r)$, proposed by some of us in Tortora et
al. (2007), able to smoothly interpolate between constant $M/L$ models and a
wide class of dark matter haloes. The resulting galaxy model is then fitted to
the data on the Einstein radius and velocity dispersion. Our phenomenological
model turns out to be in well agreement with the data suggesting the presence
of massive dark matter haloes in order to explain the lensing and dynamics
properties of the SLACS lenses. According to the values of the dark matter mass
fraction, we argue that the halo may play a significant role in the inner
regions probed by the data, but such a conclusion strongly depends on the
adopted initial mass function of the stellar population. Finally, we find that
the dark matter mass fraction within $R_{eff}$ scales with both the total
luminosity and stellar mass in such a way that more luminous (and hence more
massive) galaxies have a larger dark matter content.
