The mass of matter in the universe that cannot be observed by direct observations of its emitted or absorbed electromagnetic radiation. There are a number of astrophysical observations that suggest that the actual mass of the universe is much greater than that estimated by observations using optical telescopes, radiotelescopes, etc. It is thought that there is a considerable amount of dark matter (or hidden matter) causing this discrepancy. Various explanations have been put forward for the missing mass, including black holes, brown dwarfs, cosmic strings, axions, neutrinos, monopoles, and various exotic particles, such as weakly interacting massive particles (WIMPS), which are predicted to exist by supersymmetry. A less conventional alternative explanation of the observations that gave rise to the idea of missing mass is modified Newtonian dynamics. The universe contains far more missing matter than directly observable matter. It has also been suggested that some dark matter could be in the form of Kaluza–Klein particles, i.e. particles postulated to exist in certain theories of fundamental physics, such as Kaluza–Klein theory, in which there are more than four space–time dimensions.