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
Invisible matter helps to hold the Universe together.
An astrophysicist looks for dark matter in an underground lab.
Astrophysicist Dan Bauer looks for dark matter in a lab half a mile below the plains of Minnesota. This unusual location shields his equipment from other particles that might interfere with his search for dark matter particles, which are predicted to pass straight through the Earth. If this is correct, Bauer may be able to detect and verify the existence of this mysterious form of matter.
A mysterious form of matter is predicted to exist.
Although no-one knows what type of particles make up dark matter, scientists predict it exists. They base their predictions on their observations of the motion of stars in other galaxies.
Scientists are puzzled by missing matter.
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]
Vera Rubin's star velocity measurements support theory.
Measurements of the velocities of stars orbiting in galaxies made by Dr Vera Rubin in the 1970s are evidence of dark matter's existence. Dark matter is thought to provide the "extra gravity" needed to reconcile the orbits of stars with Newton's law of gravity.
Scientists hunt for elusive particles in a Yorkshire mine.
Professor Tim Sumner explains how he hunts for elusive dark matter particles in Boulby mine in Yorkshire.
Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but would account for most of the matter in the universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. Other than neutrinos, a form of hot dark matter, it has not been detected directly, making it one of the greatest mysteries in modern astrophysics.
Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known 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, while dark energy plus dark matter constitute 95.1% of the total mass–energy content of the universe.
Astrophysicists hypothesized dark matter because of discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the observable matter (stars, gas, and dust) that they can be seen to contain. Dark matter was postulated by Jan Oort in 1932, albeit based upon flawed or inadequate evidence, 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. Adequate evidence from galaxy rotation curves was discovered by Horace W. Babcock in 1939, but was not attributed to dark matter. The first to postulate dark matter based upon robust evidence was Vera Rubin in the 1960s–1970s, using galaxy rotation curves. Subsequently many other observations have indicated the presence of dark matter in the universe, including 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, some alternative theories of gravity have been proposed, such as MOND and TeVeS, which try to account for the anomalous observations without requiring additional matter. However, these theories cannot account for the properties of galaxy clusters.