Using the Hubble Space Telescope, astrophysicist Richard Massey at England’s Durham University and colleagues identified
some 30 magnified and warped images of a distant background spiral galaxy that appeared in the core of a nearby cluster of galaxies. Through gravitational lensing, the spiral galaxy’s light essentially photobombed the cluster and, in doing so, illuminated how matter — both light and dark — is distributed within the cluster and around one of its four central galaxies.
“We noticed that the dark matter seems to have become separated from [at least one of the central galaxy’s] stars,” says Massey. “That implies that it was acted upon by different forces, and followed a slightly different trajectory. The most plausible option … is the dark matter in at least one of the galaxies is feeling a frictional force from the dark matter in the cluster.” That is, dark matter is pushing against itself, a totally unexpected behavior for the ethereal material.