Galactic smashup leaves dark matter debris

Find in ‘train wreck’ cluster forces re-thinking of theories about mysterious substance

Amid the wreckage of a massive galactic collision lies an enigmatic heart of darkness —a clump of dark matter. Normally, dark matter — the unknown substance that makes up the majority of the universe’s mass — remains anchored to galaxies. But in Abell 520, a galactic cluster sometimes known as the “train wreck,” dark matter appears to have collected in the core even though the galaxies had moved on after colliding. Using the Hubble Space Telescope, researchers inferred the blob’s presence based on how it distorts light reaching Earth, they report in the March 10 Astrophysical Journal. Located 2.4 billion light-years from Earth, the parked clump challenges theories proposing that galaxies and dark matter travel together. Previously, a different galactic smashup called the Bullet Cluster supported the idea of tandem travelers. But now, astronomers seeking to explain the train wreck observations are considering alternative theories and scenarios, including the presence of sticky dark matter or dim galaxies hiding in the dark heart.

GALATIC SMASHUP This composite image captured by the Hubble shows the positions of the dark matter core (blue), galaxies (orange) and gas (green) in the train wreck cluster, formed by colliding galaxies. NASA, ESA, CFHT, CXO, M.J. Jee/UC Davis, A. Mahdavi/San Francisco State Univ.

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