- Proposed large amount of matter present in galaxies but not visible. It has been proposed that 90–98 per cent of the mass of many galaxies may be ‘missing,’ especially on the basis of studies of the orbits of stars around galactic centres. But other lines of investigation, including hunts for the gravitational lensing effects which such mass ought to produce, imply that reports of its existence may have been greatly exaggerated. It has been variously proposed that the missing mass may exist in the haloes above and below the galaxies’ planes of rotation and that it may be present in the form of exotic atomic particles, or exist as MACHOs, planet-sized objects in deep space. The presence of this material, if it exists, has immense cosmological importance, especially for the expansion and possible future contraction of the universe.