The two types of mass measurement for galaxies give very different answers. Those based on the effects of gravity (dynamics of stars determined from measurements of redshift) are ten times greater than those inferred from brightness. The mass-to-light ratio for the Sun is 1; the mass-to-light ratio for galaxies is ~10. The mass-to-light ratio increases to ~100 when galactic halos are included; and rises to ~300 for clusters of galaxies.
Standard reasoning asserts that, since bright matter underestimates the amount of mass in a galaxy, this 'missing mass' must be present in a 'dark' form.
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