A galaxy isn't big just because it has many stars in it. A worldwide research group has discovered that a galaxy in the Coma cluster, Dragonfly 44, consists of 99.99 percent dark matter. It has about as much mass as our own Milky Way galaxy, but far fewer stars. The team determined the presence of the invisible, mysterious substance based on the motions of the stars themselves -- there were too few of them to be moving so quickly. If there weren't a gravitational force like dark matter to hold them together, those stars would simply fly away.