Although scientists know the masses of the sun and Earth, it's a different story for the galaxy. Mass estimates range widely: At the low end, some studies find that the galaxy is several hundred billion times as massive as the sun whereas the largest values exceed two trillion solar masses. Astronomers would have an easier task if the galaxy consisted solely of stars. But a huge halo of dark matter engulfs its starry disk and vastly outweighs it. Now remarkable observations of a small galaxy orbiting our own have led to a new number.