Dark matter is the invisible substance or substances thought to make up the majority of the matter in the universe. Unlike ordinary matter, dark matter does not give off, reflect, or absorb light rays. Scientists have detected dark matter through the effects of its gravitational pull on visible objects. Scientists have estimated that about 85 percent of the matter in the universe is dark matter.
The Swiss astrophysicist Fritz Zwicky first argued for the existence of dark matter in 1933. Zwicky was studying a cluster of galaxies called the Coma Cluster. The galaxies moved around one another too quickly to be held together by the gravitation of their visible matter alone. Zwicky inferred that additional gravitation from invisible matter must help hold the cluster together. Further evidence came from other scientists, including Vera C. Rubin and W. Kent Ford, Jr., of the United States. These scientists observed irregularities in the motion of stars within galaxies. These irregularities also imply large amounts of invisible matter in galaxies. More recent evidence of dark matter comes from a variety of sources, including studies of radiation left over from the big bang (explosive beginning of the universe).
When scientists first proposed the existence of dark matter, many of them considered it likely to consist of large objects that give off little light. Such objects might include white dwarf stars, neutron stars, black holes, brown dwarfs, or large planets. Dark matter researchers call such objects MACHOs (massive compact halo objects). However, researchers have found that there are too few MACHOs to account for the majority of dark matter.
Today, most astrophysicists think that the majority of dark matter consists of undiscovered subatomic particles (particles smaller than atoms) called WIMPs. WIMP stands for weakly interacting massive particle. WIMPs interact only through the force of gravity and through another fundamental force called the weak nuclear force. At one time, scientists thought that WIMPs might be neutrinos, a type of subatomic particle without an electric charge. But computer simulations have shown that neutrinos move too quickly to hold galaxies and galaxy clusters in such compact shapes. However, slower-moving particles, known as cold dark matter, could provide the extra gravity to form more compact structures. Many scientists think the neutralino, a hypothetical WIMP, is a leading candidate for cold dark matter.
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Scientists have devised many experiments to look for evidence of WIMPs. Most particles from space cannot penetrate Earth, but WIMPs can. By placing particle detectors deep underground, researchers hope to observe WIMPs without contamination from other particles. Other experiments use telescopes or other detectors to search for dark matter. These instruments look for particles that may be created when a pair of WIMPs come into contact and destroy each other. This process, known as WIMP annihilation, could give off gamma rays and neutrinos and other energetic particles that might prove the existence of WIMPs. Physicists also hope to create WIMPs in particle accelerators, such as the Large Hadron Collider in Switzerland.
Although much dark matter research focuses on WIMPs, they are not the only undiscovered particles that have been proposed to make up dark matter. One such particle is the axion, a hypothetical, electrically neutral particle with a small mass.
Current theories concerning dark matter are based on Albert Einstein’s general theory of relativity. This theory underlies the scientific understanding of gravity. If the theory is incorrect, however, dark matter may not exist. Some thinkers have tried to replace Einstein’s theory with theories that eliminate the need for dark matter. Such efforts have had little success.
