SCIENTISTS have found a “ghost” galaxy which is roughly the same mass as our own but entirely made up of dark matter.
Dragonfly 44 is almost entirely made up of dark matter, the mysterious stuff that makes up 27 percent of the universe, but has never actually been seen.
Though the galaxy is relatively nearby, at least in the scale of the universe, it is so dark that scientists completely missed it for decades.
But it was finally spotted last year. It sits in the Coma galaxy cluster, about 330 million light years from us.
When scientists looked at it further, they found that it was not just a normal set of stars, but instead a ghost, made up of dark matter. Though it has about the same mass as our own Milky Way galaxy, only one hundredth of one percent is made of up of the normal matter like stars, dust and gas that surrounds us.
Rather, it is 99.99 percent made up of dark matter. Nobody knows what exactly that is, how it came about, or even how a galaxy could have arisen that looked that way.
Dragonfly 44 does have some normal stars of its own, but our galaxy has 100 times more stars than are there.
Astronomers found out about the strange ghost galaxy by looking at the movement of the galaxy's stars – movement that seemed to be influenced by matter that doesn't exist by normal measures.
Professor Pieter van Dokkum, a member of the team from Yale University in the US, said: “Motions of the stars tell you how much matter there is.
"They don't care what form the matter is, they just tell you that it's there.
“In the Dragonfly galaxy, stars move very fast, so there was a huge discrepancy.
“We found many times more mass indicated by the motions of the stars than there is mass in the stars themselves.”
Scientists know there must be something providing the gravity that is needed to hold the galaxy together.
But the mass that would normally provide that isn't there.
Scientists from the Keck Observatory in Hawaii found the galaxy, and report their findings in the Astrophysical Journal Letters.
They said there may be more of the strange, ghost galaxies waiting to be found.
Dark energy makes up 68 percent of the universe, and is a kind of anti-gravitational force that is pushing galaxies apart, more and more quickly.