‘Dark’ Milky Way-sized galaxy found

Astron-omers 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. “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.