The Great Annihilator is a very powerful high energy X-ray source near the center of the Milky Way Galaxy; it appears to be a massive black hole, 10 to 1,000 times the mass of the Sun, that is producing an enormous flux of positrons (antielectrons). These antiparticles are interacting with ordinary matter nearby to produce a strong flux of electron annihilation radiation at a characteristic energy of 511 keV.