Non-baryonic matter is matter that, unlike all the kind kinds of matter with which we are familiar, is not made of baryons (including the neutrons and protons found in all atomic nuclei). Proposed as a possible constituent of dark matter, it could come in two forms, classified as cold non-baryonic matter or hot non-baryonic matter. The former would be made of particles moving much slower than light, of which there are several, as yet undetected, candidates; hot non-baryonic matter would be made of particles moving very fast, such as neutrinos. Non-baryonic matter (hot or cold) is supposed to interact weakly with radiation. Therefore, the imprints left by the non-baryonic matter in the cosmic background radiation would be different than those left by the baryonic matter. This attribute could be used to measure the contribution of non-baryonic matter to the total amount of mass in the universe.