Matter and antimatter cannot coexist, so when they meet, they annihilate each other. When an electron collides with a positron, for example, the annihilation energy is carried away by two photons, each of which has energy of 0.511 million electron volts. The total combined mass is released in a spectacular blast of energy. In a fission bomb a maximum of 0.1% of the plutonium mass can become energy; for a fusion bomb the maximum figure is about 0.5%; the figure for antimatter could approach 100%. We could call it 200%, since the explosion releases the energy of our 1 kilogram of antimatter plus that of the 1 kilogram of matter with which it reacts.
Electrons and positrons, having opposite charge together vanish into high-energy gamma rays (plus a certain number of harmless neutrinos, which pass through whole planets without effect). Hitting ordinary matter, our 1 kilogram of antimatter explodes with the force of up to 43 million tonnes of TNT — as though several thousand Hiroshima bombs were detonated at once. In Star Trek, the starship Enterprise is propelled by a controlled version of such explosions, but the television series never explained how the antimatter was created or stored.