Dark matter may exist in an exotic previously-unknown form according to Harvard University physicists. The new type of dark matter, called double-disk dark matter (DDDM), is not predicted by current models, but may play a crucial role in the formation of galaxies, and possibly even form dark matter galaxies.
The Milky Way and other galaxies similar to it are roughly disk-shaped due to the actions of gravity and spin on the stars that comprise the systems. Beginning as a spinning balls of gas, energy is lost from collisions between particles, beginning the process of collapse. As the spinning gas becomes denser, it flattens out into a pancake-like shape. Dark matter, as it is currently understood, does not fall into this shape, because collisions between dark matter particles are too rare to dissipate enough energy to begin the collapse. Instead, it surrounds galaxies like a halo.
Current models of dark matter propose that it is composed of a single type of particle, that rarely collides with any other particle because it has so little energy. The new form of dark matter proposed, however, behaves more like ordinary mater in that sense. The collisions could dissipate enough energy to start dark matter collapsing.
"It can form a disk which is very similar to our galaxy's massive galactic disk." Andrey Katz, post-doctoral student at Harvard University and co-author of the paper announcing the results, said.
This strange form of dark matter could make up as much as five to ten percent of the total mass of dark matter in the Universe.
"We are saying, yes, we know dark matter is in mostly the form of a spherical halo," said Harvard's Matthew Reece, another co-author of the paper. "But there could be different kinds of dark matter, and maybe 10 percent of it forms disks. That constraint is compatible with what we observe."
These strange galaxies would only be able to form if there also existed other strange phenomenon as well, including dark matter forms of electromagnetic radiation, including light. It is the production and release of this dark-matter energy that would allow the material to radiate enough energy to start the collapse into a disk.
The existence of DDDM could be detected from its influence on the motion of stars in galaxies. If the substance creates gamma rays, if the model allows, then this energy would fill the sky with a distinctive signal that could be measured.
This new form of dark matter was proposed in the May 23 issue of Physical Review Letters.
