schwit1 writes: New research by astronomers suggests that the Milky Way is about half as massive as previously estimated. It was thought to be roughly the same mass as Andromeda, weighing in at approximately 1.26 x 10^12 solar masses (PDF). This new research indicates its mass is around half the mass of Andromeda. "Galaxies in the Local Group are bound together by their collective gravity. As a result, while most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity. For the first time, researchers were able to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda. ... Andromeda had twice as much mass as the Milky Way, and in both galaxies 90 percent of the mass was made up of dark matter."