"The most powerful potential energy source presently thought to be available to humanity, antimatter is a term normally heard in science-fiction films and TV shows, whose heroes fly "antimatter-powered spaceships" and do battle with "antimatter guns."
But antimatter itself isn't fiction; it actually exists and has been intensively studied by physicists since the 1930s. In a sense, matter and antimatter are the yin and yang of reality: Every type of subatomic particle has its antimatter counterpart. But when matter and antimatter collide, they annihilate each other in an immense burst of energy. " (AntiMatter Weapons)
"Unlike regular nuclear bombs, positron bombs wouldn''t eject plumes of radioactive debris. When large numbers of positrons and antielectrons collide, the primary product is an invisible but extremely dangerous burst of gamma radiation. This can kill a large number of soldiers without touching the civilian population." (Anti-matter weapon systems million times more powerful than nukes - reverse engineering extraterrestrial technologies)