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Galactic split provides clue to dark matter mystery

Separation of stars from invisible mass suggests existence of self-interacting particles

7:05pm, April 14, 2015
Cluster Abell 3827

GALACTIC SMASH-UP  The many galaxies of the cluster Abell 3827 appear in this Hubble Space Telescope photograph. The center of the cluster contains four colliding galaxies (yellow blobs). The bluish circle around the galaxies is the light from a more distant galaxy bending around the cluster’s center due to gravity from both stars and dark matter.

A cosmic collision has somehow separated a galaxy from its dark matter, the mysterious invisible stuff that typically dominates a galaxy’s mass. The dark matter may be lagging behind its host galaxy because another clump of dark matter slowed it down. If so, it would be the first evidence that dark matter interacts through a force other than gravity.

“It’s exciting to wonder if it could be dark matter interacting with itself,” says Neal Weiner, a theoretical physicist at New York University.

The study, published April 15 in Monthly Notices of the Royal Astronomical Society, is the latest using galactic collisions to probe the behavior of dark matter. By charting dark matter interactions in the cosmos, physicists hope to narrow down theorists’ extensive list of candidates for dark matter’s identity.

Decades of research show that

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