Re: Why are astrophysicists looking for only one source of dark matter?

Hi Matt,

You're exactly right that there might be multiple kinds of dark matter. It would be much simpler if there were only one type of dark matter, and Occam's Razor suggests that we should look for the simplest solution first. However, recent evidence suggests that the universe is not so simple. If gravity works, then there has to be much more mass in the haloes of galaxies than can be accounted for by the stars we see. This dark matter is not necessarily anything too exotic; it could be composed of very dim objects similar to those we already know about, such as white dwarfs, neutron stars, or free-floating Jupiter-mass objects. Since these objects are massive and compact, they can bend light and produce gravitational lensing. Many groups have tried to find evidence for dark matter made up of these MACHOs (MAssive Compact Halo Objects), and they have had some success. According to some recent results, between 8 and 50% of the halo of the Milky Way, our own galaxy, may be made up of these MACHOs, which are a form of dark matter. While this is more than would be expected based on counting stars, these scientists are fairly certain that there are not enough MACHOs to provide all of the missing mass in the Milky Way's halo.

Also, there are some arguments that there is more mass in the universe than can be accounted for by baryonic matter, 'ordinary' matter like stars and planets that is made of protons and neutrons. The present day abundance of helium and other elements formed just after the big bang suggests a specific density of baryonic matter in the universe. This density is lower than that inferred from studies of the masses of galaxies and clusters of galaxies, which again suggests that dark matter may not be made up of (only) MACHOs.

The above arguments lead us to more exotic candidates for dark matter, usually called WIMPs, or Weakly Interacting Massive Particles. These particles are thought to interact only weakly with ordinary matter, much like neutrinos do. So, is the rest of the dark matter made up of massive neutrinos? Maybe, but there is no solid evidence as yet that neutrinos have any mass. Recent experiments suggest that neutrinos may have a very small mass, possibly much too small to account for the rest of the dark matter. Theorists are good at thinking of other possibilities, including particles called neutralinos, photinos, and axions.

There are even subclasses of WIMPs. Theorists characterize two types as 'hot' dark matter and 'cold' dark matter (though some scientists are now talking about 'warm' dark matter). Cold Dark Matter candidates like neutralinos have larger masses and lower velocities than Hot Dark Matter candidates like neutrinos. Computer simulations suggest that it is difficult to form the observed large-scale structure in the universe if the dark matter is Hot, so the current favorite is Cold Dark Matter (CDM), although there may be small amounts of Hot (or even Warm!) dark matter sprinkled in. There are multiple experiments in progress that are trying to directly detect CDM particles, but again, there is no indisputable proof just yet that they exist. Hopefully, one or more of these experiments will detect a new class of WIMPs and solve one of the biggest mysteries in astronomy.

Further reading:
Kaufmann, Universe
Alcock, C. Science 2000, vol. 287, p.74
Trimble, V. Annual Reviews of Astronomy and Astrophysics 1987, v.25, p.425
Rubin, V. Scientific American 1998
A Dark Matter site at UC Berkeley with more links