Gamma rays are high-energy electromagnetic radiation (in excess of 100,000 electron volts) that can be generated by nuclear reactions in space, originally discovered as an emission of radioactive substances. Gamma ray bursts are brief bursts of gamma rays from the distant universe, observed by satellites. There exists no generally accepted explanation for gamma ray bursts. Some promising theories were abandoned when NASA's Compton Gamma Ray Observatory satellite (CGRO) found in 1991 that they seemed to occur equally in all directions. Had they originated in our own galaxy, they would have probably been concentrated in the direction of the Milky Way, where most of our galaxy's stars are found (the galaxy is a flattened disk, and when we look at the Milky Way we see it edge-on). The new evidence suggests that they could come instead from distant galaxies, and if so, their sources must be incredibly powerful.