Access: Shedding Light on Dark Matter

The cold, the warm, and the hot

Current theory describes dark matter in three ways. Cold dark matter (CDM) is highly interactive gravitationally with other matter and has no relativistic speed. That is, it is not moving at a percentage of the speed of light. Computer simulations show that CDM results in a universe much like the one we observe if the simulation is done on a very large, multigalactic scale. However, at smaller scales CDM doesn't fare quite as well, creating simulated galaxies with too many small dense clumps of matter.

Matter density in an area 10 megaparsecs on each side. The image on the left shows matter density as it would occur if dark matter adhered to the properties of cold dark matter (CDM). At right, is the same area with matter density as it would occur in warm dark matter (WDM) theory. WDM produces empty voids, whereas CDM results in many small objects inside these spaces.
Click each image to enlarge it.

"Cold dark matter predicts many dwarf structures (small, rogue galaxies) between the large clumps in a galaxy," says Bode. "That is not what has been observed. "

Another possible form of dark matter—hot dark matter—describes a particle like massive neutrinos, which are so weakly interactive that they pass right through regular matter. Scientists have been able to detect neutrinos using huge particle detectors, and neutrinos are believed to be a type of dark matter. However, if most dark matter were neutrinos, matter in the universe would be quite uniformly distributed, explains Bode, and researchers know that it is not.

Ostriker and Bode simulate the formation of galaxy halos using the warm dark matter (WDM) model. WDM is a slight variation of CDM. Because particle velocities are lower, dark matter clumps more with its surroundings than do neutrinos but not as much as CDM particles.