Element zero, also known as eezo, is a substance used in Mass Effect for the titular mass effect technology. In theory, it is an element with an atomic number of zero, meaning that the nucleus of an eezo atom does not contain a proton. Just what particles comprise an eezo atom isn't clear.
When exposed to an electrical current, element zero releases dark energy to create a mass effect field. In their most basic form, mass effect fields either increase or decrease the mass of matter held within or simulate the effects of a gravity field. A wide variety of complicated applications exist for element zero, most notably faster-than-light travel. In utero exposure to dust-form element zero can cause living beings to develop latent mass effect field manipulation abilities, called biotics.
Element zero is generated when solid matter, such as a planet, is affected by the energy of a star going supernova. The material is common in the asteroid debris that orbits neutron stars and pulsars. These are dangerous places to mine, requiring extensive use of robotics, telepresence, and shielding to survive the intense radiation from the dead star. Eezo can also be found in significant quantities among the ruins of extinct spacefaring civilizations.