Gamma rays are extremely powerful, high intensity electromagnetic radiation, produced by subatomic annihilation. Gamma ray bursts have been observed from explosions in other galaxies. These massive bursts of energy are a major subject of study in astronomy, representing extraordinary levels of radiation. Pulsars and quasars are known sources of gamma rays. Gamma rays are said to be so powerful they can cross whole galaxies, and pass through all but major mass bodies. Gamma radiation is very common in the universe, and mapping of astronomical macro structures has produced a map showing a band of gamma sources.