A particular type of non-baryonic particle that, according to some theories, is created in the early stages of the Big Bang and survives to the present time in sufficient numbers to contribute significantly to the present density of the Universe. The term ‘hot’ signifies that these particles are fast-moving (close to the speed of light), usually because they are of low mass. The favoured candidate for such a particle is a neutrino with a rest mass of around 10 eV, which is 1/500 000 of the mass of the electron.