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Galactic collision creates mysterious 'dark core'

Images captured by the Hubble telescope reveal a mysterious clump of dark matter thought to be left behind after a massive galactic collision. But this dark matter isn't behaving in the way scientists expect dark matter to behave. 

This composite image, captured by NASA's Hubble Space Telescope and the Canada-France-Hawaii Telescope in Hawaii shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters.

NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)

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Astronomers are left scratching their heads over a new observation of a “clump” of dark matter apparently left behind after a massive merger between galaxy clusters. What is so puzzling about the discovery is that the dark matter collected into a “dark core” which held far fewer galaxies than expected. The implications of this discovery present challenges to current understandings of how dark matter influences galaxies and galaxy clusters.

Initially, the observations made in 2007 were dismissed as bad data. New data obtained by the Hubble Space Telescope in 2008 confirmed the previous observations of dark matter and galaxies parting ways. The new evidence is based on observations of a distant merging galaxy cluster named Abell 520. At this point, astronomers have a challenge ahead of them in order to explain why dark matter isn’t behaving as expected.

“This result is a puzzle,” said astronomer James Jee (University of California, Davis). “Dark matter is not behaving as predicted, and it’s not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing.”

Current theories on dark matter state that it may be a kind of gravitational “glue” that holds galaxies together. One of the other interesting properties of dark matter is that by all accounts, it’s not made of same stuff as people and planets, yet interacts “gravitationally” with normal matter. Current methods to study dark matter are to analyze galactic mergers, since galaxies will interact differently than their dark matter halos. The current theories are supported by visual observations of galaxy mergers in the Bullet Cluster, and have become a classic example of our current understanding of dark matter.


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