Dark Matter Explained

Evidence for Dark Matter

The first compelling evidence for dark matter came from astronomer Fritz Zwicky in 1933, who studied the velocities of galaxies in the Coma Cluster and found they were moving far too fast to be held together by the visible matter alone. He proposed that some unseen mass, which he called dunkle Materie, must be providing the additional gravity needed to keep the cluster bound. His observations were largely ignored for decades until Vera Rubin and Kent Ford provided similar evidence in the 1970s through their studies of galaxy rotation curves.

Rubin and Ford measured the speeds at which stars orbit the centers of spiral galaxies and found that stars in the outer regions move just as fast as stars closer to the center. This flat rotation curve contradicts what would be expected if only the visible matter were present, since orbital speeds should decrease with distance from the center according to Newtonian gravity. The simplest explanation is that each galaxy is embedded in a large, roughly spherical halo of dark matter that extends well beyond the visible disk, providing the additional gravitational pull needed to keep the outer stars in orbit.

Gravitational lensing provides another powerful line of evidence. When light from a distant galaxy passes near a massive foreground object, the gravity of the foreground object bends the light, distorting and magnifying the background image. By analyzing the degree of distortion, astronomers can map the distribution of mass in the foreground object, including mass that cannot be seen. Studies of galaxy clusters through gravitational lensing consistently show that the total mass far exceeds what can be accounted for by visible matter and hot gas, with the excess attributed to dark matter.

The cosmic microwave background provides perhaps the most precise evidence. The patterns of temperature fluctuations in the CMB can be analyzed to determine the relative amounts of ordinary matter, dark matter, and dark energy in the universe. Results from the Planck satellite show that ordinary (baryonic) matter makes up about 5 percent of the universe, dark matter about 27 percent, and dark energy about 68 percent. Without dark matter, the observed CMB pattern and the formation of galaxies and large-scale structure cannot be explained.

What Dark Matter Could Be

The leading candidates for dark matter fall into two broad categories: particle dark matter and modified gravity theories. Most physicists favor particle dark matter, hypothesizing that dark matter consists of one or more types of subatomic particles that interact with ordinary matter primarily through gravity and possibly through the weak nuclear force, but not through electromagnetism.

Weakly Interacting Massive Particles (WIMPs) have been the most popular candidate for decades. WIMPs would have masses between about 10 and 1,000 times that of a proton and would interact with ordinary matter only through gravity and the weak force. If WIMPs existed with the right mass and interaction strength, they would naturally produce the observed amount of dark matter in the early universe, a coincidence known as the WIMP miracle. Despite extensive searches using underground detectors, particle colliders, and space-based observatories, no definitive WIMP detection has been made.

Axions are another leading candidate. Originally proposed to solve a problem in quantum chromodynamics (the strong CP problem), axions would be extremely light particles, potentially trillions of times lighter than an electron. Axion dark matter experiments use strong magnetic fields to convert axions into detectable photons, and several next-generation experiments are currently searching for them. Other candidates include sterile neutrinos, primordial black holes formed in the early universe, and various particles predicted by extensions of the Standard Model of particle physics.

Modified gravity theories, such as Modified Newtonian Dynamics (MOND), attempt to explain the observations attributed to dark matter by modifying the laws of gravity at very low accelerations rather than invoking a new type of matter. While MOND can successfully reproduce many galaxy rotation curves, it struggles to explain observations at larger scales, such as the dynamics of galaxy clusters and the CMB, without introducing some form of additional mass. Most cosmologists consider dark matter particles to be the more likely explanation, but modified gravity remains an active area of theoretical research.

Dark Matter and Cosmic Structure

Dark matter plays a fundamental role in the formation of cosmic structure. In the early universe, dark matter began to clump together under gravity before ordinary matter did, because dark matter does not interact with light and was therefore not affected by the radiation pressure that kept ordinary matter smooth and hot. Dark matter halos formed first, and then ordinary matter fell into these gravitational wells, cooling and condensing to form stars and galaxies. Without dark matter, galaxies and galaxy clusters would not have formed in the time available since the Big Bang.

Computer simulations of cosmic structure formation, such as the Millennium Simulation and IllustrisTNG, use the properties of dark matter to predict how the cosmic web of filaments, clusters, and voids should look. These simulations produce results that match the observed large-scale distribution of galaxies remarkably well, providing strong support for the cold dark matter model. In this model, dark matter particles move slowly compared to the speed of light (hence cold), allowing them to clump on all scales from dwarf galaxies to superclusters.

However, some discrepancies remain between simulations and observations at small scales. Simulations predict more small satellite galaxies around large galaxies like the Milky Way than are actually observed (the missing satellites problem), and they predict that the centers of dark matter halos should be denser than observations suggest (the core-cusp problem). These issues may be resolved by better accounting for the effects of ordinary matter on dark matter distributions, by invoking dark matter particles with slightly different properties, or by future observations that discover more faint satellite galaxies.