In the 1970s, an astronomer called Vera Rubin was measuring the velocities of stars in other galaxies and noticed something strange: the stars at the galaxies' edges moved faster than had been predicted. To reconcile her observations with the law of gravity, scientists proposed that there is matter we can't see and called it dark matter.
Physicists are racing to find subatomic particles that could be the missing dark matter, which is thought to make up about 26% of the energy density of the Universe.
Image: A computer-generated image of dark matter's potential distribution across millions of light years of space
The particle detectors used for the UK Dark Matter Collaboration experiment are housed in a mine deep under the North Yorkshire Moors. The BBC's David Shukman finds out what the scientists are hoping to learn.
Hubble Space Telescope images provide evidence of dark matter's existence. Light from distant galaxies is bent by a gravitational lens created by the dark matter's mass in nearby galaxies.
Professor Tim Sumner explains how he hunts for elusive dark matter particles in Boulby mine in Yorkshire.
Sir Patrick Moore and his guests explain what galaxies are and discuss some of their interesting features.
In the 1970s, Professors James Peebles and Jeremiah Ostriker's computer model simulations of galaxies suggested that there are large amounts of unaccounted for matter in the Universe. However, their ideas did not gain wider acceptance until Vera Rubin's measurements of the speeds of stars in galaxies also suggested that there is missing matter, which is now known as dark matter. ["The size and mass of galaxies and the mass of the universe" copyright Ostriker and Peebles-The Astrophysical Journal 193 / "Dark Matter and the origin of galaxies and globular star clusters" copyright Peebles - The Astrophysical Journal 277: 470-477]
In astronomy and cosmology, dark matter is a type of matter hypothesized to account for a large part of the total mass in the universe. Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. Instead, its existence and properties are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe.
Dark matter came to the attention of astrophysicists due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and the mass calculated from the "luminous matter" they contain: stars, gas and dust. It was first postulated by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way, and by Fritz Zwicky in 1933 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, many other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies by Vera Rubin, in the 1960s–1970s, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, the temperature distribution of hot gas in galaxies and clusters of galaxies, and more recently the pattern of anisotropies in the cosmic microwave background. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics today.
Although the existence of dark matter is generally accepted by the mainstream scientific community, there is no generally agreed direct detection of it. Other theories including MOND and TeVeS, are some alternative theories of gravity proposed to try to explain the anomalies for which dark matter is intended to account.
On 3 April 2013, NASA scientists reported that hints of dark matter may have been detected by the Alpha Magnetic Spectrometer on the International Space Station. According to the scientists, "The first results from the space-borne Alpha Magnetic Spectrometer confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays."
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