A new galaxy as big as the Milky Way has been discovered, except it’s 99.99 per cent dark matter

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 Thursday 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.

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. 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. 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.”