The largest scale on which the mass density has been measured with any
precision is that of superclusters. A supercluster is an aggregate of several
clusters of galaxies, extending over about 10 Mpc. Our local supercluster is
an extended distribution of galaxies centered on the Virgo cluster, some 10-20
Mpc distant, and our Milky Way galaxy together with the Andromeda galaxy forms
a small group (the Local Group) that is an outlying member of the Virgo
Supercluster. The mass between us and Virgo tends to decelerate the recession
of our galaxy, as expected according to
by about ten percent.
This effect is seen as a deviation from the uniform Hubble expansion of the
galaxies and provides a measure of the mean density within the Virgo
Supercluster. One again finds a ratio of mass-to-luminosity equal to 300 over
this scale, which amounts to about 20Mpc.