Dark Matter Composition

Let's be clear -- we don't know the exact nature of dark matter. But we can look at some possibilities.

First, dark matter could be ordinary matter, which is made of protons, neutrons and electrons. This ordinary matter does not emit or absorb light, but it does exert gravitational effects. Some possibilities include the following:

• Brown dwarfs -- These large objects formed in the same way as stars but never accumulated enough gas and dust to reach the critical mass to start hydrogen fusion (see How Stars Work, How the Sun Works). Brown dwarfs are about 5 percent of the sun's mass, i.e. usually larger than a planet, but not as large as a star. Astronomers call these and similar objects MACHOs, which stands for Massive Compact Halo Objects. MACHOs can be detected by gravitational lensing. Astronomers think that brown dwarfs are not numerous enough to account for dark matter in the galaxy.
• White Dwarfs -- These are the remnants of the cores of dead small to medium-size stars (see How Stars Work). Although large numbers of white dwarfs exist, there are not enough to account for dark matter (there should be large amounts of leftover helium from them, but this has not been observed).
• Neutron stars/black holes -- These are the last remnants of the cores of large stars after supernovae explosions (see How Stars Work, How Black Holes Work). While they do ­have large gravitational effects and are invisible because they can even prevent light from escaping (black holes), they are far too rare to account for dark matter.

Second, dark matter might be an entirely new type of matter, or extraordinary matter. Extraordinary matter probably consists of subatomic particles that interact weakly with ordinary matter and have been called WIMPs (for Weakly Interacting Massive Particles).

• Neutrinos -- Subatomic particles that move near the speed of light, but have little mass. These particles probably make up little dark matter within galaxies because they move fast enough to escape even a galaxy’s pull of gravity. However, they may constitute some dark matter between galaxies. So, it is doubtful that they make up much dark matter.
• New subatomic particles -- There could be many of these proposed particles. Many come from the theory of supersymmetry, which doubled the number of particles from the standard model (see ­How Atom Smashers Work). They move relatively slowly and are relatively cold (i.e. undetectable by infrared and X-ray telescopes). Particle physicists are actively trying to find evidence for these theoretical particles to explain dark matter.
• Neutralinos (massive neutrinos) -- hypothetical particles that are similar to neutrinos, but heavier and slower. Although they have not been discovered, they are a leading candidate for extraordinary dark matter. Axions -- small, neutral, low-mass (less than a millionth of an electron) particles Photino -- similar to photons, but have a mass that is 10 to 100 times greater than a proton. Photinos are uncharged and interact weakly with matter.

Scientists estimate that ordinary matter might make up to 20 percent of the dark matter in the universe. ­