Dark matter, identified in the 1930s, cannot be detected directly because it doesn’t seem to emit or absorb light. But astronomers know it must be there because they see its gravitational effect on the light and visible matter around it. And since there’s about five times more dark matter than normal matter, its gravitational pull has a profound effect on the shape of the universe.
It’s relatively easy to see when light is bent by the massive, dense clumps of dark matter that fill galaxy clusters. But the thin, sparse dark-matter filaments thought to connect them simply don’t bend as much light and thus have proved difficult to find.
Using data from the Subaru telescope on Mauna Kea in Hawaii and the XMM-Newton space telescope, researchers examined 40,341 background galaxies for signs of distortion. They used a model to subtract out the masses of the galaxy clusters and then fit the remaining mass with a model of what a filament might look like. They found that a filament must be present, and that it was actually much longer than it seemed.