The Milky Way – our own, sometimes very visible galaxy – seen over Northumberland International Dark Sky Park Alan Wallace
Scientists have found a “ghost” galaxy – 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 – and for now mostly theoretical – stuff that makes up 27 per cent 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 per cent is made of up of the normal matter like stars, dust and gas that surrounds us.
Rather, it is 99.99 per cent 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 a hundred 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 by normal measures exist.
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 that 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 that there may be many more of the strange, ghost galaxies waiting to be found.
“The … 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.”
Dark matter remains perhaps the biggest mystery of the universe. While scientists know that it must exist – the calculations that account for the make-up of the universe require it – we’ve never actually seen it, and attempts to do so have failed.
But the discovery could let us finally find more about the mysterious stuff that surrounds us.
“Ultimately what we really want to learn is what dark matter is,” said Dr Van Dokkum.
“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.”
Only 5 per cent of the interchangeable mass-energy of the universe is made up of the kind of normal matter that we can see and touch. Dark matter makes up a large part of the rest.
Despite the fact that it constitutes 27 per cent of the universe, it doesn’t reflect light and can’t be seen by any means so far. Experiments to understand it usually require doing so through other means – but even they have often failed.
The remaining part of the universe is made by something even more confusing. Dark energy makes up 68 per cent of the universe, and is a kind of anti-gravitational force that is pushing galaxies apart, more and more quickly.
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