A Mystery in Gamma Rays . ... When the EGRET instrument on the orbiting Compton Gamma-ray Observatory surveyed the sky in the 1990s, it cataloged 271 celestial sources of high-energy gamma-rays. ... But 170 of the cataloged sources, shown in the above all-sky map, remain unidentified. Many sources in this gamma-ray mystery map likely belong to already known classes of gamma-ray emitters and are simply obscured or too faint to be otherwise positively identified. ... NASA Official: Jay Norris. ...