The Enormous Size of the Milky Way.
A recent study has summarized various methods to determine the mass of the Milky Way, including measuring the motion of stars, globular clusters, satellite galaxies, tidal plumes of dwarf galaxies, and stars leaving the galaxy. The team took a statistical average of these methods and determined the best value for the mass of the galaxy to be a trillion solar masses, give or take a few hundred billion solar masses. These methods rely on orbital motion and have varying levels of accuracy, but they all suggest that most of the galaxy's mass is in the form of dark matter.
