Suppose one measures the total mass of a galaxy somehow and its total luminosity. What mass to light ratio (in solar units) is considered a signal for dark matter and why? For example if M(galaxy)/L(galaxy) = 10 M(sun)/L(sun), is that considered 'too high' and a signal of dark matter? My question is related to the fact that there are many stars that have higher mass-to-light ratio than the sun. Why would we expect the total galactic mass-to-light ratio be of the order of the sun's if there is no dark matter present?