  Motivated by present indirect evidences that galaxies are surrounded by dark
matter halos, we investigate whether their physical properties can be described
by a formulation of the virial theorem which explicitly takes into account the
gravitational potential term representing the interaction of the dark halo with
the barionic or luminous component. Our analysis shows that the application of
such a ``two-component virial theorem'' not only accounts for the scaling
relations displayed, in particular, by elliptical galaxies, but also for the
observed properties of all virialized stellar systems, ranging from globular
clusters to galaxy clusters.
