Pym Particles are the mass-shifting atomic particles developed/discovered by Henry Pym that allow him to change size and assume the identity of Ant-Man.
Pym has developed the means to weaponize the particles, and has used them to shrink dangerous threats to manageable size. When he used them on Bruce Banner during a tense battle against his alter-ego the Hulk, the Particles reacted with Banner’s Gamma-charged cells to cause a bizarre and unexpected chain reaction. Referred to as The Nevada Incident, it resulted in the death of General ‘Thunderbolt’ Ross, the release of significant amounts of Gamma radiation over a large patch of the Nevada desert, and, most significantly, the complete disappearance of Banner. He is presumed to have been shrunk out of existence, and Pym has spent much of his time since trying to develop instruments to locate Banner.