A recently discovered galaxy is about 99.99% dark matter, hinting at new insights into galaxy formation and dark matter’s mysteries.
At first glance, Dragonfly 44—the galaxy in the center of the two images above—seems insignificant, even difficult to notice. (It was only discovered in 2015.) It lacks the iconic, central bulge of fully-formed galaxies, like our own Milky Way. Yet, stars are swirling around Dragonfly 44 at speeds similar to stars within the Milky Way, meaning that is has a mass similar to the Milky Way, but only one percent as many stars.
Writing in Astrophysical Journal Letters, a team of astronomers suggest that the rest of the galaxy's mass is dark matter, making Dragonfly 44 a failed galaxy. Dragonfly 44 is a type of galaxy known as ultra diffuse galaxies. Recent astronomical studies have suggested that such galaxies, which often have masses far greater than the combined mass of their stars, are “failed” galaxies, never producing the number of stars their mass would suggest possible.
Dragonfly 44 is unusual among known ultra diffuse galaxies, because it is similar to mass to our own Milky Way. Galaxies like the Milky Way are usually very efficient at forming stars in a well understood process, yet Dragonfly 44 challenges that understanding.
Additionally, astronomers may find Dragonfly 44 to be a promising target for further study of dark matter. This large mass of dark matter is located only 300 million light-years from Earth, within the Local Cluster—our universal backyard.