Here's a Knot of Dark Matter Binding Two Galaxies Together

No one has ever caught a dark matter particle, or really knows what it is. But, based on its bizarre effects in our universe—the way it causes galaxies to rotate, for example—physicists believe that dark matter is there, knitting everything together in a vast web. Since we've never observed it directly, we have to go on indirect observation, and faith: Dark matter makes up about a quarter of the entire universe.

Now scientists at the University of Waterloo have produced the first-ever composite image of part of what that huge dark matter web looks like. By measuring light travelling from a distant galaxy, and watching as it warped around other galaxies and material on its way to Earth, they could indirectly detect and image a dark matter filament binding two galaxies together.

"We've detected the filament," author Mike Hudson, a cosmologist and professor of astronomy, told me over the phone. "But we haven't detected the dark matter particle."

The technique they used is called weak gravitational lensing, he explained. As light travels from a distant object in the universe towards us, "it gets deflected a bit," he said, creating a distortion in the shape of the background object. "If the background object is a perfectly round galaxy," he continued, "it would be distorted into a small ellipse."

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Using images from a multi-year sky survey at the Canada-France-Hawaii telescope, Hudson and co-author Seth Epps watched as light warped around a pair of galaxies and the dark matter filament that binds them. "The combination of these two galaxies, and the filament in between them, caused the distortion of more distant galaxies," he said.

Other teams have produced  maps of dark matter using gravitational lensing before. But what's different here, Hudson told me, is that he and Epps have made a composite image of a dark matter filament—one that's based on actual data—and it can be used to learn more about the filament's properties, including width and mass. "It's not a theoretical model," he said. "It's coming from measurements of the real universe."

Are all galaxies connected to each other by similar threads? "We think that most of them probably have faint connections to each other, of differing degrees of strength," he said. "The Milky Way may be embedded in a filament with its neighbour, Andromeda."

Here on Earth, scientists are still searching for the dark matter particle with a variety of detectors. For now, though, observing its indirect effects in the stars seems to be more productive. Hudson thinks that particle searches are equally important.

"We would love to see the particle itself," he said. These searches keep turning up empty-handed, I pointed out. "We could be in the unfortunate situation where its interaction strength is just very weak," he said. "Maybe we will never detect it through particle physics methods, and only learn about dark matter through astronomy and cosmology."

For now, the best place to search might be up in the sky.

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