Dark matter is a theoretical form of matter that cannot be observed by telescopes due to its weak interactions with electromagnetic forces. Both the existence and properties of dark matter are inferred from observations of its radiation emissions as well as its gravitational effects on matter and its function in galactic structure.
Dark matter is theorized to exist because of flat galactic rotation curves, or the velocity objects in a galaxy remaining mostly constant once a certain distance from the galactic core is reached; the observed flat galactic rotation curves are indicative of the presence of additional matter that cannot yet be directly observed.
Current cosmological models of the universe estimate that 26% of the universe is comprised of dark matter. Galaxies, made of ordinary matter, are held together by the attractive gravitational force of dark matter and are pushed apart from each other by the repulsion of dark energy, the unidentified 70% of the universe responsible for expansion.
Cosmologists make an analogy between the universe and a loaf of raisin bread in an oven. The loaf itself represents the universe, and raisins stand for individual galaxies. As the bread expands, the space between the raisins increases, but the size of the raisins remains the same.
After the Big Bang, universal expansion began, and matter started to spread from an initial state of high concentration to spreading out and forming galaxies like the Milky Way, our home. Cosmologists agree that the universe is constantly and rapidly expanding due to the repulsive forces of dark energy. While the universe as a whole is expanding, and galaxies are moving farther apart from each other, the constituents of these galaxies are remaining cohesive and are unaffected by the repulsion that drives the galaxies apart.
There must be an attractive force within these galaxies to keep their matter in place and counteract the repulsive force between galaxies to prevent intergalactic matter from flying through space. Researchers have recorded and analyzed data concerning the amount of cosmic microwave background radiation present in today’s universe. Theorized values to account for the radiation emissions of only ordinary matter in the universe were lower than the observed amounts of radiation in the universe. With the additional consideration of dark matter radiation emissions, the amounts of radiation observed aligned well with the theorized emission amounts of both ordinary and dark matter in the universe.
The discovery of dark matter is unprecedented, so there is no pre-discovered particle to compare with the properties of dark matter, and there are infinite possibilities for its identity. Using observed properties such as attractive gravitational force and elusivity, cosmologists have theorized several identities to pair with the properties of dark matter.
The leading contenders are Weakly Interacting Massive Particles (WIMPs — I think that’s a fun name). WIMPs interact weakly with both the electromagnetic and the strong nuclear forces. WIMP-WIMP annihilations would likely occur in their highest concentrations at the centers of galaxies, and energy in the form of gamma radiation as well as neutrinos are among the products of these self-annihilations.
By means of detection experiments both on earth and in orbit, the properties of dark matter are observed. Specific particles are considered in the identification of dark matter based on inferred properties of dark matter from these experiments, including those at IceCube, Large Underground Xenon Experiment, the Alpha Magnetic Spectrometer, and the Large Hadron Collider.
One cubic kilometer of digital optic modules are located underground at the South Pole to detect annihilation products of Weakly Interacting Massive Particles (WIMPs) emitting photons as they interact with ice as part of detection experiments at IceCube Neutrino Observatory.
At LUXE, a tank of liquid xenon surrounded by water located approximately a mile beneath the Black Hills of South Dakota is used to directly identify WIMPs by their occasional release of photons due to interactions with the neutrons of xenon ions.
The Alpha Magnetic Spectrometer on the International Space Station is used to observe cosmic ray events associated with positron emissions from intergalactic WIMPs.
The theory of unbroken supersymmetry states that for every fermion, or matter constituent, there exists a corresponding boson, or force carrier, with the same mass and internal quantum numbers aside from spin, where the values differ by one half-odd integer.
The correct abundance of dark matter in the universe requires a self-annihilation cross section that a particle in the 100 GeV range that interacts with the unified electroweak force would have; if two identical particles that interact with the electroweak force and have masses of about 100 GeV were to come together and annihilate, the energy their interaction would release would fall into a probable region with an area in accordance with the amount of dark matter there is in the universe. Unbroken supersymmetric extensions of the standard model indicate that such a particle is the superpartner of an ideal WIMP candidate. *