The ratio of the mass of an object to its total luminosity. The mass and luminosity are usually measured in terms of solar mass and solar luminosity; hence the mass-to-light ratio of the Sun is 1. Most extragalactic objects have mass-to-light ratios greater than 1, indicating that not all their mass is in the form of visible stars. The high values inferred for galaxies (up to 30) and clusters of galaxies (up to 300) indicates the existence of considerable amounts of dark matter.