We show that the small scatter around the Fundamental Plane ( FP ) of massive elliptical galaxies can be used to derive important properties of their dark and luminous matter .
The central velocity dispersion \sigma _ { 0 } , appearing in ( e.g . )
the Fundamental Plane , is linked to photometric , dynamical and geometrical properties of ( luminous and dark ) matter .
We find that , inside the effective radius R _ { e } , the matter traced by the light must largely dominate over the dark matter ( DM ) , in order to keep the ellipticals close enough to the FP .
This recalls analogous findings for spiral galaxies .

In particular we also find that cuspy DM distributions , as predicted by numerical simulations in \Lambda CDM cosmology , are unable to explain the very existence of the FP ; in fact , according to this theory , the structural properties of dark and luminous matter are so interwoven that a curved surface is predicted in the log–space ( \sigma _ { 0 } , R _ { e } , L ) , rather than a plane .
In order to agree with the FP , CDM halos must have concentrations parameters in the range of 5 - 9 ( i.e .
values significantly lower than the current predictions ) .

Assuming a more heuristic approach and allowing for cored DM halos , we find that the small intrinsic scatter of the FP yields to i ) an average value for the dark–to–light–traced mass ratio inside the length–scale of light R _ { e } of about 0.3 , ii ) a mass–to–light ratio of the matter traced by the light increasing with spheroid luminosity : M _ { sph } / L _ { r } \propto L _ { r } ^ { 0.2 } in Gunn– r band , with a value of 5.3 at L _ { \ast r } \equiv 2.7 \times 10 ^ { 10 } L _ { r \odot } .