Previous evidence points to dark matter not being able to interact with other dark (or regular) matter except weakly through gravity. Researchers at UBC took a look at a "cosmic train wreck" that occurred when two galatic superclusters smashed into one another. These galatic superclusters, each containing hundreds or even thousands of individual galaxies, are mostly made up of free Hydrogen gas. During the collision the gas interacts and gets "stuck" in the middle while the stars, planets, and dark matter should keep on moving through.
In this case it appears that the dark matter gets "stuck" in the middle as well, something that was not seen in previous observations of other cosmic train wrecks. There are a number of possible explanations, all of which will teach us something new about nature of dark matter:
- Some dark matter may actually be able to interact with other dark matter via some new force.
- There may have already been a large chunk of dark matter, without much regular matter, sitting in the middle where the collision took place. In this case, we need to understand how so much dark matter can exists without much regular matter.
- Dark matter has a filament-like nature, similar to how roots on a tree grow. In this case, we may be looking at one of the filaments end on. Imagine looking at a pencil end on. All you would see is the pencil point and not have any idea how long the pencil actually is. A similar thing could be happening with the dark matter–what we see as a small dense core in the center could actually be a an incredibly long filament of dark matter.
This result provides another puzzle piece in the mystery of what dark matter is, how galaxies form, and what our place in the universe is.