UNIMAGINABLE POWER

If equal masses of matter and antimatter are brought together, then in theory all the mass will be converted to energy. If there happens to be more matter than antimatter, there will be some matter left over after the encounter. Conversely, if there is more antimatter than matter, there will be some antimatter remaining.
In a nuclear reaction, only a tiny fraction of the mass of the constituents is liberated as energy; plenty of matter is always left over, although its form has changed. You might push together two chunks of ²³⁵U, the isotope of uranium whose atomic mass is 235 amu, and if their combined mass is great enough, an atomic explosion will take place. However, there will still be a considerable amount of matter remaining. We might say that the matter-to-energy conversion efficiency of an atomic explosion is low.
In a matter–antimatter reaction, if the masses of the samples are equal, the conversion efficiency is 100 percent. As you can imagine, a matter– antimatter bomb would make a conventional nuclear weapon of the same total mass look like a firecracker by comparison. A single matter–antimatter weapon of modest size could easily wipe out all life on Earth.