An image of the galaxy clusters Abell 222 and Abell 223 connected by a dark… (Joerg Dietrich, University…)
The mysterious stuff called dark matter is thought to act as the spider silk for the cosmic web of the universe. But although it makes up most of the matter in the universe, scientists have been able to find only clumps of it in the web's galaxy-filled "nodes," not along the gossamer threads that are thought to help give the universe its structure.
Now, after much searching and with a little luck, an international team of astrophysicists has discovered a dark matter filament connecting two clusters of galaxies about 2.7 billion light-years away.
The discovery, published online Wednesday by the journal Nature, catapults these filaments from sound theory to observable fact.
"I have to say the evidence is pretty strong," said Manoj Kaplinghat, a theoretical astrophysicist at UC Irvine who was not involved in the study. "There have been other claims that have sort of gone away, but this one looks like the best one I've seen. As far as I can tell, this really is the first."
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.
"Dark matter really governs structure formation," said study leader Joerg Dietrich, an astrophysicist at the University Observatory Munich in Germany. "The galaxy clusters and the filaments are mostly made up of dark matter. The normal matter just follows the distribution of dark matter."
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.
Previous studies that purported to identify these filaments were later shown to be false or inconclusive.
But Dietrich's team discovered a rare find: two clusters of galaxies positioned very close to each other on the plane of the sky, making it easier to look for a filament between them.
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.
From our point of view on the flat sky, the filament appears to be a short and dense connector a mere 3 million light-years or so thick. But in reality, the filament is long and thin, as filaments are thought to be: about 58 million light-years from end to end.
It's a matter of perspective, Kaplinghat explained: If you were to position a dumbbell so that one end was almost right behind the other, it would look as if the two ends were just an inch apart, even though the bar connecting them might be half a foot long.
The light reaching us is bent by all the dark matter along the 58-milion-light-year length of the filament. All that mass bending the light over our line of sight makes for a stronger signal — much better than if the scientists had seen the filament from a side-view angle.
"In a sense, we were lucky," said study coauthor Aurora Simionescu, an astrophysicist at Stanford University.
Marusa Bradac, a UC Davis astrophysicist who was not involved in the study, said the next step would be to try to find more of these filaments to ensure that this was not a "pink elephant." But Bradac cheered the findings.
"I was really happy that this had finally happened," Bradac said. "We've been predicting dark matter filaments to exist for ages.... It's just excellent to be able to see them for real now."