Much of the universe is made of dark matter, the unknowable, as-yet-undetected stuff that barely interacts with the "normal" matter around it. In the Milky Way, dark matter outnumbers regular matter by about 5 to 1, and very tiny dwarf galaxies are known to contain even more of the stuff.
But now scientists have found something entirely new: a galaxy with the same mass as the Milky Way but with only 1 percent of our galaxy's star power. About 99.99 percent of this other galaxy is made up of dark matter, and scientists believe it may be one of many.
The galaxy Dragonfly 44, described in a study published 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.
The researchers who found Dragonfly 44 weren't looking for a dark galaxy. Another surprise: They found it using a telescope built of camera parts. The Dragonfly Telephoto Array was built by a group of astronomers at Yale University and the University of Toronto who realized that telephoto lenses — so 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," study author Pieter van Dokkum of Yale told The Post. At first, van Dokkum and his colleagues thought they were seeing image defects. But when they looked more closely at their data, they realized they'd found an entirely new class of object.
Let's pause here to remind ourselves that scientists have no idea what dark matter is. Dark matter is the name given to a mysterious form of matter that we know must be there. Without something adding to the mass of the universe, things would tear apart. The laws of physics require some unknown, unseen kind of something to hold the universe together (while dark energy pushes it farther and farther apart). All we really know about dark matter is that it barely interacts with anything, including its own ilk.
Dragonfly 44 wasn't like the dark galaxies that have been found before, which are all quite tiny. It was as big as the Milky Way, but with far fewer stars.
"They are so diffuse, these galaxies, so tenuous, that they would be ripped apart. There just wasn’t enough mass to hold them together," van Dokkum said.
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 galaxy to be sure of its mass. What they found shocked them.
"It's pretty crazy, the difference from the Milky Way is a factor of 100," he said. It's as if someone picked through the Milky Way, selecting just one star out of 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."
And unlike in previously observed galaxies where dark matter dominates only in certain areas — in the Milky Way, the dark-matter fraction is less than half that of the star-filled central regions — it rules all of Dragonfly 44. Even in the central, visible regions, dark matter makes up about 98 percent of the total mass.
What's "especially bothersome," according to van Dokkum, is the fact that the galaxy is roughly the same size as the Milky Way — 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."
"We thought that that ratio of matter to dark matter was something we understood. 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," he continued.
"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 closer to home. 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 quiet enough for scientists to pick out the first direct evidence of dark matter.
The researchers are confidently searching the sky. "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."
In fact, he's hopeful that scientists will eventually be able to detect even darker galaxies — and perhaps even some that are made up entirely of dark matter.