National Aeronautics and Space Administration

White Dwarfs are what remain of a small to medium sized star after it has passed through the red giant phase.

	Pros:	There is an abundance of white dwarfs in the universe. If young galaxies produced white dwarfs that cool more rapidly and become undetectable, maybe they could be abundant enough to explain dark matter.
	Cons:	With the production of huge numbers of white dwarfs, in theory, one would expect to see the production of massive amounts of helium. However, this is not observed.

Brown Dwarfs have a mass that is less than eight percent of the mass of the Sun, resulting in a mass too small to produce the nuclear reactions that make stars shine. The signature of these objects is an occasional brightening.

	Pros:	Astronomers have observed distant objects that are either brown dwarf stars or large planets around other stars. Astronomers believe that the brightening and dimming of brown dwarfs are due to the gravitational lens effect of a foreground star. They also believe the brightening and dimming may provide further evidence for a large population of brown dwarfs in our Galaxy.
	Cons:	While they have been observed, astronomers have found no evidence of a large population of brown dwarfs that would account for the dark matter in our Galaxy.

WIMPs (Weakly Interacting Massive Particles): WIMPs are the little, weak, subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Particle physicists search for WIMPs. Examples: Exotic subatomic particles such as axions, heavy neutrinos, and photinos.

	Pros:	Theoretically, there is the possibility that very massive subatomic particles, created in the right amounts, and with the right properties in the first moments of time after the Big Bang, are the dark matter of the universe.
	Cons:	Observations have been fruitless. No one has observed even one of these particles.

Hydrogen Gas

	Pros:	Hydrogen gas, one of the most basic elements, is 70-75% of the visible matter in the universe. Most of the dark matter may exist as small clouds of hydrogen gas.
	Cons:	Hydrogen is easily detected by radio, infrared, optical, ultraviolet, and X-ray telescopes. The necessary amount of hydrogen hasn’t been seen.