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'Dark Core' May Not Be So Dark After All

Douglas Clowe of Ohio University in Athens, Ohio, is reporting on new Hubble observations that do not find an unusually dense clump of dark matter in the universe that a different Hubble team reported on earlier this year. The region of interest lies at the center of a collision among massive galaxy clusters in Abell 520, located 2.4 billion light-years away.
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'Dark Core' May Not Be So Dark After All

Astronomers were puzzled earlier this year when NASA's Hubble Space Telescope spotted an overabundance of dark matter in the heart of the merging galaxy cluster Abell 520. This observation was surprising because dark matter and galaxies should be anchored together, even during a collision between galaxy clusters.
Astronomers have abundant evidence that an as-yet-unidentified form of matter is responsible for 90 percent of the gravity within galaxies and clusters of galaxies. Because it is detected via its gravity and not its light, they call it "dark matter."
Now, a new observation of Abell 520 from another team of astronomers using a different Hubble camera finds that the core does not appear to be over-dense in dark matter after all. The study findings were published in The Astrophysical Journal.

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Dark Matter Core Defies Explanation in Hubble Image

Astronomers observed what appeared to be a clump of dark matter left behind during a bizarre wreck between massive clusters of galaxies. The dark matter collected into a "dark core" containing far fewer galaxies than would be expected if the dark matter and galaxies hung together. Most of the galaxies apparently have sailed far away from the collision. This result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to the invisible substance, even during the shock of a collision.
The initial observations, made in 2007, were so unusual that astronomers shrugged them off as unreal, due to poor data. However, new results obtained in 2008 from NASA's Hubble Space Telescope confirm that dark matter and galaxies parted ways in the gigantic merging galaxy cluster called Abell 520, located 2.4 billion light-years away. Now, astronomers are left with the challenge of trying to explain dark matter's seemingly oddball behaviour in this cluster.

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