In a Cosmology Breakthrough, Astronomers Measure a Filament of Dark Matter

Dark Matter Filament This image shows the galaxy clusters Abell 222 and Abell 223 connected by a dark matter filament. The blue shading and the yellow contours indicate matter density. This image on the sky is about twice as big as the full moon. Jörg Dietrich, University of Michigan/University Observatory Munich

Invisible, cold dark matter plays a major role in the evolution of galaxies, according to modern cosmological theory. The most advanced simulations of cosmic evolution show stringy tendrils of mass — dark matter — connecting giant clusters of galaxies via a vast cosmic web. Now for the first time, astronomers have been able to detect one of these filaments, sussing out its location by watching it warp light.

In a new paper, Jörg Dietrich and colleagues report a dark matter filament attaching the Abell 222-223 supercluster system. The team used the Subaru telescope on Mauna Kea to observe the galaxy clusters, located 2.7 billion light years away in the Cetus constellation. Any light coming toward Earth from behind this supercluster will be warped and magnified by its mass, a phenomenon known as gravitational lensing. From our perspective, distant galaxies look like distorted funhouse-mirror smears.

In this new study, Dietrich and colleagues noticed that the cluster’s observable mass, including stars and hot X-ray-emitting gases, could not account for all this lensing — it was only about 9 percent. Something else with a lot of mass must also be in the way. The only possible explanation is dark matter, which is so called because we can’t see it.

By examining the lensing, which is weak compared to other massive systems, the team was able to figure out the dark matter filament’s location and precise shape. This was only possible because of the way Abell 222 and 223 are arranged on the sky relative to Earth — they look close together from our perspective, allowing a straight-on view. Calculating the location of all the cluster’s mass, there’s a clear bridge connecting Abell 222 and 223.

This is a big breakthrough for cosmology, because it strengthens the case for dark matter’s existence. It also shows that it may be possible to determine exactly where it is and how it’s arranged.

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