Dark matter

Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass–energy density. Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. Dark matter does not interact with the electromagnetic force, meaning it does not absorb, reflect, or emit light, making it extremely difficult to detect directly.

Evidence for existence

The primary evidence for dark matter comes from calculations showing that many galaxies would fly apart, or that they would not have formed or move as they do, if they did not contain a large amount of unseen matter. Other lines of evidence include observations of gravitational lensing—where light from distant galaxies is bent by the gravitational influence of dark matter—and the cosmic microwave background radiation, which shows the distribution of matter in the early universe. The rotational speeds of galaxies, the velocity dispersions of galaxies in clusters, and the distribution of hot gas in galaxy clusters all point toward the existence of dark matter.

Composition and properties

The exact nature of dark matter remains unknown, but it is believed to be composed of one or more types of subatomic particles that interact very weakly with ordinary matter. The leading candidate is a hypothetical particle known as a weakly interacting massive particle (WIMP). Other possibilities include axions, sterile neutrinos, and other exotic particles. Dark matter is cold, meaning it moves slowly compared to the speed of light, which is consistent with the observed large-scale structure of the universe. It does not emit, absorb, or reflect light, and its presence is inferred solely through its gravitational effects.

Detection methods

Efforts to detect dark matter can be divided into three main categories: direct detection, indirect detection, and collider production. Direct detection experiments aim to observe the rare interactions of dark matter particles with ordinary matter in highly sensitive underground detectors. Indirect detection methods look for the products of dark matter particle annihilations or decays, such as gamma rays, neutrinos, or other cosmic rays. Collider experiments, like those at the Large Hadron Collider, attempt to create dark matter particles in high-energy particle collisions. To date, no definitive direct or indirect detection has been confirmed, though several experiments have reported potential signals that require further investigation.

Role in structure formation

Dark matter plays a crucial role in the formation and evolution of cosmic structures. In the current cosmological model, the Lambda-CDM model, dark matter provides the gravitational scaffolding for the formation of galaxies, galaxy clusters, and larger-scale structures. After the Big Bang, slight density fluctuations in the dark matter distribution grew over time due to gravity, pulling ordinary matter along to form the first stars and galaxies. Without dark matter, the universe would lack the necessary mass to explain the observed clustering and distribution of galaxies, as well as the detailed features of the cosmic microwave background.

Alternatives to dark matter

While dark matter is the prevailing explanation for the observed gravitational anomalies, some scientists have proposed modifications to the laws of gravity as an alternative. Theories such as Modified Newtonian Dynamics (MOND) suggest that at low accelerations, the laws of gravity differ from those predicted by Newton and Einstein, potentially eliminating the need for dark matter. However, these alternatives struggle to explain all observational evidence, such as the bullet cluster collision, where the separation of visible matter and gravitational mass strongly supports the existence of dark matter. Most of the scientific community continues to favor the dark matter hypothesis due to its consistency with a wide range of data.