Enlarge / A compact dwarf galaxy, which may have features that are difficult to explain with standard models of dark matter. (credit: ESA/Hubble & NASA)
It is impossible for a telescope to image and far from being completely understood, yet dark matter is everywhere.
The deepest mysteries about dark matter relate to its nature and behavior. The prevailing idea regarding dark matter is the cold dark matter theory (CDM), which posits that dark matter is made up of low-velocity particles that do not interact with each other. This thinking has been debated—and it is up for debate again. Led by astrophysicist Hai-Bo Yu, a team of researchers from the University of California Riverside have come up with an alternative idea that explains two extremes where cold dark matter doesn't work well.
Galaxies and galaxy clusters are thought to be surrounded by halos of dark matter. At one end of the controversy are galactic dark matter halos that are too dense to be consistent with CDM, and at the other are galactic dark matter halos too diffuse for CDM to make sense of. Yu and his colleagues instead suggest that some dark force (sorry Star Wars fans—not the Force) causes dark matter particles to smash into each other. This is Self-Interacting Dark Matter, or SIDM.
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