The presence of dark matter is usually inferred by the way its strong gravity bends light travelling from distant galaxies that lie behind it — distorting their apparent shapes as seen by telescopes on Earth. But it is difficult to observe this 'gravitational lensing' by dark matter in filaments because they contain relatively little mass.
Dietrich and his colleagues got around this problem by studying a particularly massive filament, 18 megaparsecs (58.7 million light years) long, that bridges the galaxy clusters Abell 222 and Abell 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, explains Dietrich. The team examined the distortion of more than 40,000 background galaxies, and calculated that the mass in the filament is between 6.5 × 10^13 and 9.8 × 10^13 times the mass of the Sun.
Dark-matter filaments, such as the one bridging the galaxy clusters Abell 222 and Abell 223, are predicted to contain more than half of all matter in the Universe. Jörg Dietrich, University of Michigan/University Observatory Munich
By examining X-rays from plasma in the filament, observed by the XMM-Newton spacecraft, the team calculated that no more than 9% of the filament's mass could be made up of hot gas. The team's computer simulations suggest that roughly another 10% of the mass 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 in Cambridge, notes that astrophysicists do not know precisely how visible matter follows the paths laid out by dark matter. “What’s exciting is that 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 to discriminate between different models of how the structure formed.
Refining the technique could also help to pin down the identity of dark matter — whether it is a cold (slow-moving) particle or a warm (fast-moving) one, like a neutrino — because 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 Large Hadron Collider,” says Alexandre Refregier, a cosmologist at ETH Zurich, the Swiss Federal Institute of Technology in Zurich.
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