  We discuss the development and physics of the cosmic virial theorem and its
traditional application as an indicator of the cosmic mean density. The
standard result that the density must be sub-critical relies on the assumption
that the galaxy three-point correlation function accurately describes the mass
distribution around a typical pair of galaxies. To challenge this assumption,
we develop a simple model of this mass distribution based on the extension of
galactic halos. In such a model, one sees clearly the importance of the local
mass distribution around pairs to the prediction of the galaxy pair-wise
velocity dispersion. The model provides insight into the mechanics of the
theorem and demonstrates that a flat universe is {\it consistent} with the
observations. More generally, we conclude that the relative pair-wise velocity
dispersion probes the mass clustered around galaxies and that the observations,
viewed in this light, indicate a lower bound of $\sim 300\; h^{-1}$kpc on the
radius of galactic halos.
