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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Dark matter | 2/10 | https://en.wikipedia.org/wiki/Dark_matter | reference | science, encyclopedia | 2026-05-05T11:00:51.613917+00:00 | kb-cron |
=== 1970s === The hypothesis of dark matter largely took root in the 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in Princeton, New Jersey, by Jeremiah Ostriker, Jim Peebles, and Amos Yahil, and in Tartu, Estonia, by Jaan Einasto, Enn Saar, and Ants Kaasik.
One of the observations that served as evidence for the existence of galactic halos of dark matter was the shape of galaxy rotation curves. These observations were done in optical and radio astronomy. In optical astronomy, Vera Rubin and Kent Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. At the same time, radio astronomers were making use of new radio telescopes to map the 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen (HI) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of the Andromeda Galaxy with the 300-foot (91 m) telescope at Green Bank and the 250-foot (76 m) dish at Jodrell Bank already showed the HI rotation curve did not trace the decline expected from Keplerian orbits. As more sensitive receivers became available, Roberts & Whitehurst (1975) were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii; that paper's Figure 16 combines the optical data (the cluster of points at radii of less than 15 kpc with a single point further out) with the HI data between 20 and 30 kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic HI spectroscopy was being developed. Rogstad & Shostak (1972) published HI rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended HI disks. In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the Westerbork Synthesis Radio Telescope. In 1978, Steigman et al. presented a study that extended earlier cosmological relic-density calculations to any hypothetical stable, electrically neutral, weak-scale lepton, showing how such a particle's abundance would "freeze out" in the early Universe and providing analytic expressions that linked its mass and weak interaction cross-section to the present-day matter density. By decoupling the analysis from specific neutrino properties and treating the candidate generically, the authors set out a framework that later became the standard template for weakly interacting massive particles (WIMPs) and for comparing particle-physics models with cosmological constraints. Though subsequent work has refined the methodology and explored many alternative candidates, this paper marked the first explicit, systematic treatment of dark matter as a new particle species beyond the Standard Model. By the late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy.
=== 1980s and 90s ===
A stream of observations in the 1980–1990s supported the presence of dark matter. Persic, Salucci & Stel (1996) is notable for the investigation of 967 spirals. The evidence for dark matter also included gravitational lensing of background objects by galaxy clusters, the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background.
=== 2000s to present === Since the turn of the millennium, the search for particle dark matter has been dominated by the hypothesis of weakly interacting massive particles (WIMPs), driven by hypothesized connections to supersymmetry. Experimental efforts were characterized by a rapid increase in sensitivity using liquid xenon detectors, including XENON, LUX, PandaX, and LUX-ZEPLIN. Despite pushing interaction limits down by orders of magnitude, these direct detection experiments all reported null results for WIMPs across the standard GeV–TeV mass range. As of late 2025, the LZ experiment had excluded WIMP cross-sections above 9 GeV/c2 and reported the first detection of boron-8 solar neutrinos via coherent elastic neutrino-nucleus scattering in a dark matter detector; this marks the experimental entry into the neutrino floor "fog," an irreducible background of neutrino noise that complicates future WIMP searches. Concurrently, the failure of the Large Hadron Collider to detect supersymmetric particles has constrained the theoretical parameter space for WIMPs. These constraints have shifted significant focus toward alternative candidates such as axions. The Axion Dark Matter Experiment achieved sensitivity to the plausible DFSZ axion model in the micro-electronvolt range by the early 2020s. The prevailing view among cosmologists remains that dark matter is composed primarily of some type of not-yet-characterized subatomic particle. While this remains the majority opinion, the lack of particle detection has led to a divergence in consensus, with macroscopic candidates such as primordial black holes seeing renewed interest following observations by LIGO and JWST. The search for such particles, by a variety of means, is one of the major efforts in particle physics.
== Technical definition ==