An international team of astronomers have discovered a galaxy next door, in terms of the scale of the universe, which consists entirely of dark matter. Dragonfly 44 lies in the Coma constellation and was overlooked until last year, when it was discovered, because of its unusual composition. It's about the size of the Milky Way but with far fewer stars.

Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey

“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,” said Yale University astronomer Pieter van Dokkum.

The researchers said that velocity of stars is a good way to determine mass because the faster the stars move, the more mass its galaxy will have. “Amazingly, the stars move at velocities that are far greater than expected for such a dim galaxy. It means that Dragonfly 44 has a huge amount of unseen mass,” said co-author Roberto Abraham of the University of Toronto.

Scientists initially spotted Dragonfly 44 with the Dragonfly Telephoto Array, a telescope invented and built by van Dokkum and Abraham. They estimate the mass of Dragonfly 44 to be one trillion times the mass of the sun, which is similar to the mass of the Milky Way. However, what makes this galaxy different is that only one hundredth of 1% is in the form of stars and "normal matter". The other 99.99% is in the form of dark matter, a hypothesized material that remains unseen but may make up more than 90% of the universe.

Milky Way/Reuters

Finding galaxies composed entirely of dark matter is not new for scientists but they note that most such galaxies are dwarf galaxies, that is, 10,000 times less massive than Dragonfly 44.

“We have no idea how galaxies like Dragonfly 44 could have formed,” said Abraham. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”

Scientists said the discovery of this galaxy could essentially change the theory on how galaxies are formed. Their findings are reported in the Astrophysical Journal Letters.