Dark Matter Since the work of Fritz Zwicky in the 1930's, astrophysicists have been unable to account for all the matter in the Universe. The amount of visible matter in the Universe is less than 1% of the critical density that would be required to reverse the expansion of the Big Bang, and it is estimated from calculations of nuclear processes in the early Universe and from observations of the cosmic background radiation that at most a few per cent of the critical density could be in the form of conventional matter. Much dark matter However, it is also estimated from the motions of material in and around galaxies, as well as from clusters of galaxies, that there must be much more matter that has so far escaped direct detection. This dark matter is thought to comprise a few tenths of the critical density. It is thought that this dark (invisible) matter has played a key role in the formation of galaxies and stars, starting from the small fluctuations seen in the cosmic background radiation, that are believed to have originated during cosmological inflation. What is it? It is thought that the dark matter may be composed of massive, weakly-interacting particles. These could include neutrinos, if they are heavy enough, but current experiments do not encourage this possibility. Alternatively, the dark matter might consist of heavier particles that could be discovered at particle accelerators. One of the possible candiates for such a heavier particle is the lightest of the 'supersymmetric' particles that appear in many conjectured extensions of the Standard Model: for an introduction to this basic, but not quite simple subject, see for example this paper.

This image of the large spiral galaxy NGC 1232 was obtained with the Very Large Telescope (VLT). Its distance from Earth is about 100 million light-years. It contains more dark matter than visible matter.