An international team of astronomers discovered Dragonfly 44, a massive galaxy in the Coma constellation. They say it has the same mass as the Milky Way but 99.99 percent of Dragonfly 44 is composed of dark matter.
“Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together,” points out the study’s lead author Pieter van Dokkum, an astronomer at Yale University.
The findings, to be published in the Astrophysical Journal Letters, were made possible using the W.M. Keck Observatory and the Gemini North telescope in Hawaii. The team realized that the galaxy’s stars move at velocities greater than what is expected for a dim galaxy. Considering star velocities signify a galaxy’s mass, this means that Dragonfly 44 has an enormous amount of unseen mass.
To be exact, Dragonfly’s mass is believed to be 2 tredecillion kilograms, similar to 1 trillion solar masses. Nevertheless, only one-hundredth of one percent of the mass is made up of stars and normal matter. Dark matter is responsible for 99.99 percent of its mass.
Other galaxies whose composition is mostly dark matter have been found before. However, these galaxies are 10,000 times less massive than Dragonfly 44.
As of now, the research team, which was also composed of University of Toronto, San Jose State University, University of California Observatories and Harvard University, does not know how the new galaxy was formed. Still, they say that a huge fraction of the stars is in the form of very compact clusters, which could be an important clue to understanding Dragonfly 44.
Van Dokkum adds that the research team aims to locate other dark galaxies that are even closer to Milky Way than this recently discovered galaxy is. This could help experts understand what dark matter really is.