A ‘finger’ of the universe’s dark matter skeleton has been observed for the first time. The technique used to directly detect a slim dark matter bridge joining two galactic superclusters could eventually help astrophysicists understand the structure of the universe and identify what the mysterious invisible substance is made from.

According to the standard model of cosmology, visible stars and galaxies trace a pattern across the sky, known as the “cosmic web”, that was originally etched out by dark matter -- the unknown substance believed to account for almost 80% of the universe’s matter. Soon after the big bang, regions that were slightly denser than others pulled in dark matter, which clumped together and eventually collapsed into flat “pancakes”. “Where these pancakes intersect, you get long strands of dark matter, or filaments,” explains Jörg Dietrich, a cosmologist at the University Observatory Munich, Germany. Clusters of galaxies then formed at the “nodes of the cosmic web,” where these filaments crossed.

The presence of dark matter is usually inferred by the way its strong gravity bends light traveling from distant galaxies that lie behind it -- distorting their apparent shapes as seen in telescopes on Earth. It is very tough to observe this “gravitational lensing” by dark matter in filaments, however, because they contain relatively little mass. Dietrich and colleagues got around this problem by studying a particularly massive, 18 Megaparsec long, filament that bridged the superclusters Abel 222 and Abel 223. Luckily, this dark bridge is oriented so that most of its mass lies along the line of sight to Earth, enhancing the lensing effect, says Dietrich. The team examined the distortion of more than 40,000 background galaxies, and calculated that the mass in the filament is between about 6.5 and 9.8 x 10^13 times the mass of the sun. Their results are reported in Nature today1.

By examining X-rays from plasma in the filament2, observed by the XMM-Newton spacecraft, the team calculated that no more than 9% of the mass could be made up from hot gas. Their computer simulations suggest that roughly another 10% could be due to visible stars and galaxies. The bulk, therefore, must be dark matter, says Dietrich.

Mark Bautz, an astrophysicist at the Massachusetts Institute of Technology, notes that astrophysicists do not know precisely how visible matter follows the paths laid out by dark matter. “What’s exciting is in this unusual system we can map both dark matter and visible matter together and try to figure out how they connect and evolve along the filament,” he says. Japan’s Astro-H X-ray space telescope, due to launch in 2014, will be able to characterize the ionization state and temperature of the plasma in the filament, which will help discriminate between different models of how the structure formed.

Refining the technique could also help pindown the identity of dark matter -- whether it is a cold (slow-moving) particle or a warm (fast-moving) particle, like a neutrino -- as different particles will clump differently along the filament. The Euclid space mission, due to launch in 2019, will provide more lensing data. “This will complement direct dark matter searches, for example at the LHC,” says Alexandre Refregier, a cosmologist at the Swiss Federal Institute of Technology in Zurich.