'Dark' Milky Way-sized galaxy discovered

Astronomers have found a massive galaxy about the size of the Milky Way made almost entirely of dark matter - a material that remains unseen but is thought to make up more than 90 per cent of the universe.

The galaxy, Dragonfly 44, is located in the nearby Coma constellation and had been overlooked until last year because of its unusual composition. It is a diffuse "blob" about the size of the Milky Way, but with far fewer stars, researchers said.

"Very soon after its discovery, we realised 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 lead author Pieter van Dokkum, astronomer at Yale University in the US.

The team was able to get a good look at Dragonfly 44 using the W M Keck Observatory and the Gemini North telescope in Hawaii. Using observations taken over six nights, astronomers measured the velocities of stars in the galaxy.

They unveiled a halo of spherical clusters of stars around the galaxy's core, similar to the halo that surrounds our Milky Way galaxy. Star velocities are an indication of the galaxy's mass, researchers said. 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 Roberto Abraham of the University of Toronto. Dragonfly 44's mass is estimated to be one trillion times the mass of the Sun, which is similar to the mass of the Milky Way.

However, only one-hundredth of one per cent of that is in the form of stars and "normal" matter. The other 99.99 per cent is in the form of dark matter - a hypothesised material that remains unseen but may make up more than 90 per cent of the universe.

The researchers note that finding a galaxy composed mainly of dark matter is not new; ultra-faint dwarf galaxies have similar compositions. However, those galaxies were roughly 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," he said.

"Ultimately what we really want to learn is what dark matter is. The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle," said Van Dokkum. The research was published in the Astrophysical Journal Letters.