Christian Science Monitor reports: A quartet of colliding galaxies in a vast cluster 1.4 billion light-years away may prompt scientists to rethink their notions about the nature of dark matter – a hidden form of matter that makes up some 85 percent of all the matter in the universe.

Dark matter forms cocoons in which galaxies and clusters of galaxies form. Its gravity holds galaxies together. It’s “dark” because, as currently conceived, it rarely, if ever, interacts with ordinary matter, or even itself, other than through gravity.

Out at the cluster, known as Abell 3827, hints have emerged that dark matter may be less reclusive than previously believed. Three of the four merging galaxies appear to be sitting in the middle of their own dark-matter halos, as theory predicts. The fourth halo, however, appears to be trailing its galaxy like a reluctant retriever tugging at the end of a 5,000-light-year-long leash.

Unless astrophysicists can come up with and verify a more prosaic reason for the offset, which still could happen, this could be the first hint that dark matter does interact with other dark matter and by a means other than gravity.

If dark matter turns out to interact with itself, the implications could be profound, researchers say.

It would provide confirmation at the cosmic level that a new physics frontier lies beyond the standard model of physics, which describes a zoo of subatomic particles and their interactions. The standard model has no candidates for dark-matter particles, explains Dan Hooper, an astrophysicist at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill.

Confronted with self-interacting dark matter, physics then would have to do more than identify the subatomic particle associated with dark matter itself. They also would have to propose particles that in effect govern the interactions.

“There is a huge difference between zero interactions and even teeny tiny interactions,” explains Richard Massey, an astrophysicist at the Institute for Computational Cosmology at Durham University in Britain and the lead author of a formal description of the Abell 3827 observation, published this week in the Monthly Notices of the Royal Astronomical Society. [Continue reading…]