About eighty-five percent of the matter in the cosmos emits neither light nor any other known kind of radiation as far as is known, and hence is called dark matter. One of its other notable qualities is that it only interacts with other matter via gravity; it carries no electromagnetic charge, for example. Dark matter is also called "dark" because it is mysterious. It is not composed of atoms or their usual constituents (like electrons and protons) or of any other kind of known elementary particle.