dark matter

Hypothetical matter that, according to certain modern theories of cosmology, is thought to make up over 90% of the mass of the universe but so far remains undetected. Measurements of the mass of galaxies using modern theories showed large discrepancies in the expected values, which led scientists to the conclusion that a theoretical substance that cannot be seen had to account for a significant proportion of the universe. Dark matter, if shown to exist, would account for many currently unexplained gravitational effects in the movement of galaxies.

Theories of the composition of dark matter include unknown atomic particles (cold dark matter) or fast-moving neutrinos (hot dark matter) or a combination of both. Other theories postulate that massively dense objects such as black holes form the majority of the ‘missing’ mass in the universe.

In 1993, astronomers identified part of the dark matter in the form of stray planets and brown dwarfs, and, possibly, stars that have failed to ignite. These objects are known as massive astrophysical compact halo objects (MACHOs) and may make up approximately half of the dark matter in the Milky Way's halo.

In July 2003, the first detailed map of the dark matter distribution in a galaxy cluster was reported. An international team of astronomers studied galaxy cluster CL0024+1654, one of the largest structures in the known universe, using the Hubble Space Telescope. The cluster is mainly composed of dark matter that is only detectable by analysing the gravitational interaction between objects. The researchers targeted 39 regions of the cluster and were able to show weak gravitational lensing – gravitational warping of images of galaxies beyond the cluster caused by the presence of dark matter. The astronomers used their accumulated data to create the first dark matter map of a galaxy cluster.