A Mystery in Gamma Rays . ... What if you could see 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. ... Many sources in this gamma-ray mystery map likely belong to the already known classes of gamma-ray emitters and are simply obscured or too faint to be otherwise positively identified. ... NASA Web Site Statements, Warnings, and Disclaimers . ...