Dark matter

Dark matter is a hypothetical and invisible form of matter that does not interact with electromagnetic radiation and therefore cannot be observed directly through light. Its presence is inferred from gravitational effects that cannot be explained by visible matter alone. Across astrophysics and cosmology, dark matter is invoked to account for phenomena such as the motions of galaxies, the behaviour of galaxy clusters, gravitational lensing and the large-scale structure of the universe. Although still undetected in laboratory experiments, dark matter is widely regarded as an essential component of the standard cosmological model.

Definition and cosmological context

In contemporary cosmology, dark matter provides the gravitational framework necessary for the formation and evolution of galaxies. Soon after the Big Bang, it accumulated into dense regions along filamentary structures, creating the cosmic web in which baryonic, or ordinary, matter later collected. In the Lambda–cold dark matter (ΛCDM) model, which is the dominant paradigm, the universe’s total mass–energy budget consists of approximately 5 per cent ordinary matter, 26–27 per cent dark matter and around 68 per cent dark energy. This means dark matter accounts for about 85 per cent of the universe’s total mass.
While its density is substantial in the extended haloes surrounding galaxies, its presence in the Solar System is comparatively minimal. Estimates suggest that all the dark matter within Neptune’s orbit would have a combined mass equivalent to that of a large asteroid. Dark matter interacts with ordinary matter primarily through gravity, making direct detection using electromagnetic methods extremely difficult.
Several theoretical candidates have been proposed to explain dark matter. Weakly interacting massive particles (WIMPs) and axions are prominent examples of hypothetical particles predicted in extensions of the Standard Model of particle physics. Another possibility is that dark matter may consist of primordial black holes formed during the earliest moments of cosmological expansion.
Dark matter is often categorised by the velocities of its constituents: cold dark matter, involving particles with very low velocities; warm dark matter, with intermediate velocities; and hot dark matter, consisting of relativistic particles. Observational evidence strongly favours cold dark matter because it allows small-scale structures to form gradually through hierarchical accumulation.
While the existence of dark matter is broadly accepted, some astrophysicists argue for modifications to gravitational theory in place of, or in addition to, dark matter. Modified Newtonian dynamics (MOND), tensor–vector–scalar theories and entropic gravity are examples, but none have yet matched dark matter’s success in explaining diverse observational data.

Early history of the hypothesis

The quest to understand unseen mass in the universe spans more than a century. In the late nineteenth century, William Thomson (later Lord Kelvin) reflected on the density of stars near the Sun, speculating that many might be dark bodies invisible to observation. Henri Poincaré later used the expression “dark matter” when discussing Kelvin’s ideas, suggesting that the amount of such matter might be modest in comparison to luminous matter, though that view ultimately proved incorrect.
In the early twentieth century, several astronomers detected anomalies that hinted at unseen mass. Jacobus Kapteyn, in 1922, investigated stellar motions and surmised that additional mass might be present. Knut Lundmark’s 1930 studies suggested that the universe contained far more mass than telescopes could reveal. Jan Oort, in 1932, argued that the mass within the Milky Way’s galactic plane exceeded the amount of visible matter, although his particular calculation was later revised.
A major development came in 1933 when Fritz Zwicky applied the virial theorem to the Coma Cluster. He analysed galaxy velocities and found that the cluster’s mass would need to be hundreds of times higher than estimates based on luminous matter. This led him to infer the presence of large amounts of what he named “dark matter.” Despite inaccuracies in some of his numerical assumptions, Zwicky correctly recognised that visible matter could not account for the cluster’s gravitational effects.
Further evidence accumulated later in the twentieth century. In 1939 Horace W. Babcock reported that the rotation curve of the Andromeda Galaxy did not decrease at large radii as expected from visible mass alone, implying that its outer regions contained significant unobserved matter. Jan Oort’s 1940 study of NGC 3115 also pointed to a large, non-visible halo.

Establishment in the 1970s

Dark matter became a widely accepted concept in the 1970s, when multiple lines of observation converged. Independent research groups, one at Princeton and another in Tartu, concluded that galaxies are embedded in massive haloes of unseen matter. A pivotal line of evidence came from galaxy rotation curves. Measurements made by optical astronomers such as Vera Rubin and Kent Ford, and by radio astronomers mapping hydrogen’s 21 cm emission line, showed that rotational velocities of spiral galaxies remain constant far beyond the visible stellar disks.
These flat rotation curves contradicted expectations based on luminous matter alone. Observations of hydrogen gas at extended radii—enabled by instruments such as those at Green Bank Observatory and Jodrell Bank Observatory—made it possible to trace mass distributions much farther out than before, confirming the presence of large, diffuse haloes.
Other evidence from gravitational lensing, galaxy cluster dynamics, cosmic microwave background anisotropies and large-scale structure added further support to the dark matter hypothesis. These observations are consistent with dark matter exerting the gravitational influence required to shape cosmic structures, while remaining invisible to telescopic observation.

Contemporary interpretation and alternatives

Dark matter is central to the ΛCDM model, which successfully accounts for the universe’s evolution, from early density fluctuations to galaxy formation. The distribution of dark matter influences the patterns observed in the cosmic microwave background and the development of galaxy clusters and filaments.
Direct detection experiments, ranging from underground detectors to axion-sensitive instruments, continue to search for dark matter particles. Although no definitive detection has been made, experimental constraints have increasingly narrowed the parameter space for viable candidates.
Alternative theories propose modifying gravity instead of invoking additional matter. While these approaches can explain certain galactic-scale phenomena, they struggle to match the full spectrum of observations—particularly gravitational lensing in clusters and cosmic microwave background measurements—suggesting that even with modified gravity, some additional mass component might still be required.