A portion of the dark matter skeleton that is thought to underpin the universe's cosmic web of matter has been spotted for the first time, finally providing evidence for the theory.
Astrophysicists at the University Observatory in Munich detected a filament of the skeleton linking the two galaxy clusters Abell 222 and Abell 223, 2.7 billion light years from Earth. The evidence, gathered using data from gravitational lensing and x-rays, has been published in the journal Nature.
Astrophysics has long proposed that the web of stars and galaxies that make up the universe is supported by a cosmic scaffolding made up of fine threads of invisible dark matter. Dark matter is believed to make up more than 80 percent of all matter in the universe, so its gravitational pull acts as an important structure holding the cosmic web together. The dark matter skeleton could be described as the universe's backbone.
These threadlike strands were formed just after the Big Bang when denser portions of the universe drew in dark matter until it collapsed and formed flat disks, which featured fine filaments of dark matter at their joins. At the cross-section of these filaments, galaxies then formed.
Ordinary matter had already been located in these strands, however dark matter has always proved difficult to locate along such fine filaments. Normally, dark matter's strong gravitational pull enables researchers to locate it through gravitational lensing -- the force of its gravity bends light emitted from galaxies behind it, distorting our view of these galaxies from Earth.
However, fine filaments of dark matter have far less mass so their pull on light is not as strong, making the gravitational lensing less easy to spot.
The research team, led by astrophysicist Joerg Dietrich, therefore decided to focus on the join between Abell 222 and Abell 223 because one of its filaments is a massive 18 megaparsecs long and located in Earth's line of sight. Using data from the Subaru and XMM-Newton telescopes they used the gravitational lensing technique and calculated that the filament's mass is between 65 trillion and 98 trillion times the mass of the sun.
X-rays of the filament's plasma revealed nine percent of that mass is made up of hot gas and, according to data from a computer simulation, ten percent comes from surrounding stars and galaxies.
Everything else, the team concluded, had to be dark matter.
The results, Dietrich says, perfectly fit the skeleton theory: "We see all the three components we expect in a filament, and in the same location."
The team hope the findings will help uncover how ordinary matter and dark matter work together to construct filaments.
Astrophysicists know relatively little about dark matter, so understanding how the opposing matters work together could answer questions about its particle structure.
Two future space launches are set to gather more information on the filament -- Japan's Astro-H X-ray telescope launching in 2014 will gather data on the ionisation and temperature of plasma in the filament, and the Eudlid's 2019 launch will focus on gravitational lensing.
Further studies could help answer the questions hanging over the future of these fine strands joining the universe together -- galaxies joined by these threads have been moving towards one another, which means giant clusters with no links could map out the universe one day.