Astronomers are reportedly baffled by a “clump” of dark matter apparently left behind after a massive merger between galaxy clusters.
The cluster, observed using the Hubble Space Telescope, is known as Abell 520, resembles a “high-speed cosmic pile-up” involving multiple massive clusters of galaxies 2.4 billion light years from Earth, Arif Babul of the University of Victoria, said in a news release.
The discovery challenges the current understanding of how dark matter influences galaxies and galaxy clusters, the Christian Science Monitor wrote.
"We were not expecting this," said Babul, the study team's senior theorist in the news release. "According to our current theory, galaxies and dark matter are expected to stay together, even through a collision. But that's not what's happening in Abell 520. Here, the dark matter appears to have pooled to form the dark core, but most of the associated galaxies seem to have moved on."
Studies have shown that only about 15 percent of the matter in the universe is visible, according to NBC News, adding that most matter can be detected only by its gravitational effect.
Dark matter, which scientists suspect consists of subatomic particles (although not the same stuff as people and planets, according to the CSM), is thought to provide the invisible "scaffolding" for structure in the universe, using gravity to binding galaxy clusters into a cosmic web.
Scientists can "see" where dark matter lies by gaging the deflections of light off these clusters.
According to NBC:
"In a previous study of the Bullet Cluster, 3 billion light-years from Earth, astronomers found that concentrations of dark matter blasted through the scene of a collision, with their associated galaxies tagging along. Meanwhile, waves of hot, X-ray-emitting gas clumped up in the middle.
"In the case of Abell 520, the situation is completely different: The galaxies sailed through the collision, but the dark matter piled up in the middle, along with the hot gas."
Put another way by Wired magazine: "A 2007 study of Abell 520 showed that it was mostly typical: Wherever astronomers saw visible matter, they found a large clump of dark matter. But there was one gigantic and perplexing 'dark core' that should have attracted large amounts of visible matter yet contained almost no galaxies."
"We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped. But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other," James Jee, an astronomer at the University of California at Davis who is the lead author of the Astrophysical Journal paper, said in a news release from the Space Telescope Science Institute. "No single theory explains the different behavior of dark matter in those two collisions. We need more examples."