In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, ϒ, is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity. These ratios are often reported using the value calculated for the Sun as a baseline ratio which is a constant ϒ ☉ = 5133 kg / W : equal to the solar mass M ☉ divided by the solar luminosity L ☉, ⁠ M ☉ / L ☉ ⁠. The mass-to-light ratios of galaxies and clusters are all much greater than ϒ ☉ due in part to the fact that most of the matter in these objects does not reside within stars and observations suggest that a large fraction is present in the form of dark matter.