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An unusually high number of galaxies are aligned along a single plane running through the center of the giant Andromeda galaxy. Scientists don’t have a theory to explain why.

Galactic cannibalism or dark matter may be responsible...

Using the Hubble Space Telescope, Eva Grebel and Andrew Koch from the University of Basel in Switzerland found that nine out of Andromeda’s fourteen dwarf galaxy satellites reside in a single plane. The plane is about 52,000 light-years wide and is aligned perpendicular to Andromeda’s own galactic plane, within which the galaxy’s stars orbit about the center...

The Milky Way was found to contain two similar planes of satellite galaxies in the late 1980s, but with nothing to compare them to, astronomers couldn’t tell if such planes were a general property of galaxy formation or whether they were just a statistical fluke.

Perhaps long ago Andromeda swallowed a nearby orbiting galaxy but did a messy job of it; the galactic crumbs from that meal became Andromeda’s satellite dwarf galaxies. Such instances of galactic cannibalism are common and are believed to play a major role in galaxy formation...

Another intriguing possibility is that the satellite galaxies are actually embedded in a stream of hypothetical dark matter flowing between two massive objects.