Pieter van Dokkum and colleagues found Dragonfly 44 by mistake.
Scientists have found something entirely new: a galaxy as huge as the Milky Way that is made up almost entirely of dark matter, the mysterious stuff that barely interacts with the "normal" matter we're familiar with.
The galaxy Dragonfly 44, reported yesterday in the Astrophysical Journal Letters, is 300 million light-years away.
If scientists can track down a similar galaxy closer to home, however, they may be able to use it to make the first direct detection of dark matter.
Dark matter is the name given to something that we can't see but know must be there.
The visible matter of the universe doesn't have enough mass to hold things together. The gravitational pull of dark matter is what keeps galaxies intact.

Other galaxies made mostly of dark matter have been found before, but none were as large as this one. And it is literally dark: It has very few stars. "It's pretty crazy, the difference from the Milky Way is a factor of 100," said study author Pieter van Dokkum of Yale.In the Milky Way, there is five times more dark matter than regular matter. Dragonfly 44, in contrast, is 99.99 per cent dark matter.
It's as if someone picked through the Milky Way, selecting just one star out of every 100 and throwing the rest away. For the galaxy to stay in one piece, it must make up the difference with dark matter.
"That's just something we never knew could happen."
Van Dokkum and his colleagues were not looking for a dark galaxy. They found it using a telescope built out of camera parts. The Dragonfly Telephoto Array was built by a group of astronomers at Yale and the University of Toronto who realised that telephoto lenses - often used for nature photography and sporting events - were well-suited for spotting the kind of large, dim objects that pose problems for typical telescopes.
"We planned to study the outskirts of galaxies to see what exists around them, but by accident we saw all these little smudges,"van Dokkum told the Washington Post.
At first, van Dokkum and his colleagues thought they were seeing defects in their equipment. But when they looked more closely at their data, they realised they had found an entirely new class of object.
The researchers then turned from their maverick telescope to an old standard: the Keck Observatory in Hawaii, home to the largest telescope in the United States. They spent six nights imaging the strange galaxy to be sure of its mass.
What's "especially bothersome", according to van Dokkum, is the fact that Dragonfly 44 is a solidly average-size galaxy, the kind we thought we understood well.
"If it's a very big or very large galaxy, you can brush it off and say, 'oh, that must be a rare thing'," he said. "But most of the stars in the universe live in galaxies this size."
He continued: "We thought the formation of stars was kind of related to how much dark matter there is, and Dragonfly 44 kind of turns that idea on its head. It means we don't understand, kind of fundamentally, how galaxy formation works."
Van Dokkum hopes his team will soon find more of these galaxies. Dark matter interacts with itself very little, but scientists predict that when it does interact, it produces tiny signatures of ultraviolet light. In a typical galaxy, these signals are drowned out by the noise of stars and other matter around them. If it were close enough to study, a galaxy that is almost totally dark might be dim enough for scientists to pick out the first direct evidence of dark matter.
The researchers are searching the sky now. "We think that this galaxy is representative of an entirely new class of object," van Dokkum said. "It's not some weird singular galaxy that's just a curiosity." He hopes scientists will eventually be able to detect even darker galaxies - perhaps some that are made up entirely of dark matter.

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