DARK MATTER, hypothetical form of matter that does not reflect, absorb, or radiate light or interact with the electromagnetic force. The presence of dark matter is implied by gravitational effects that cannot be explained by the general theory of relativity unless more matter is present than can be detected. The effects of dark matter are often found when observing the motion and structure of galaxy clusters; the mass, position, and velocity of stars; and cosmic microwave background anisotropies. Researchers estimate that dark matter makes up approximately 30.1 percent of the matter-energy composition of the universe and outweighs visible matter by a ratio of sixty to one. The other 69.9 percent of the universe is composed of dark energy (69.4 percent) and visible matter (0.5 percent).
What was originally referred to as “nonluminous matter” was first detected in 1844, when Friedrich Wilhelm Bessel concluded that decades of positional measurements of the stars Sirius and Procyon showed that they must be in orbit with an “invisible companion” of a mass comparable to their own. Several decades later, Dutch astronomer Jan Oort found that the stars near the Sun left 30 to 50 percent of the expected mass implied by their velocities unaccounted for. And in 1933 Swiss astronomer Fritz Zwicky reached similar conclusions when he discovered that “the velocity dispersions in rich clusters of galaxies required 10 to 100 times more mass to keep them bound than could be accounted for by the luminous galaxies themselves.” Oort’s results in particular—dubbed the “Oort limit”— acted as a catalyst for further investigations into this phenomenon in the astrophysics community.
In the 1970s American astronomers Vera Rubin and W. Kent Ford observed a similar phenomenon, finding that the mass of stars visible within galaxies accounts for only 10 percent of the mass required to keep the stars orbiting the galaxy’s center. Dark matter’s existence has been further proved through the process of observing gravitational lensing, in which matter alters the direction of light passing by the bending of space within its gravitational field. The Chandra X-ray Observatory has also documented how the hot gas in the Bullet Cluster is slowed by the drag of one cluster passing through the other. That the mass of the clusters remains unaffected indicates that the majority of the mass of clusters consists of dark matter.
Though dark matter is incredibly difficult to observe directly, scientists have been able to distinguish between two kinds: baryonic and nonbaryonic. Baryonic dark matter makes up about 4.5 percent of the universe and is composed of the same baryons (protons, neutrons, and atomic nuclei) as observable light-emitting matter. This baryonic dark matter is expected to exist in gas forms between the galaxies. It is estimated that 26.1 percent of the universe’s matter is in the unfamiliar form of nonbaryonic dark matter. Observations and calculations indicate that nonbaryonic dark matter is relatively “cold” or “nonrelativistic,” meaning that it is composed of heavy, slow-moving particles. Little is known about the precise structure of this type of dark matter, as its particle composition does not appear to sync with the standard model of particle physics. These particles are commonly referred to as weakly interacting massive particles (WIMPs). Some theories have proposed extending the standard model of particle physics to include WIMPs, and there is currently extensive research and experimentation involving particle accelerators to attempt to determine the properties of these unseen particles.
Alternatives to dark matter, such as modifications to the theory of gravity, have been proposed to account for the presence of “missing matter,” but such theories have been unable to explain many of the observed phenomena.