Editing Dark matter (section)

=== Velocity dispersions ===
{{Main|Velocity dispersion}}
Stars in bound systems must obey the [[virial theorem]]. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies<ref>{{cite journal |last1=Faber |first1=S. M. |last2=Jackson |first2=R. E. |date=1976 |title=Velocity dispersions and mass-to-light ratios for elliptical galaxies |journal=The Astrophysical Journal |volume=204 |pages=668–683 |bibcode=1976ApJ...204..668F |doi=10.1086/154215}}</ref> do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.<ref>{{cite book |last1=Binny |first1=James |last2=Merrifield |first2=Michael |year=1998 |title=Galactic Astronomy |publisher=Princeton University Press |pages=712–713}}</ref>

As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.