What happens when matter meets antimatter?

The answer

When a particle meets its antiparticle (such as an electron meeting a positron), they annihilate each other completely, converting all their mass into energy as gamma rays via Einstein's E=mc². This makes antimatter the most energy-dense substance possible - one gram of antimatter annihilating with one gram of matter would release about 180 terajoules, equivalent to about 43 kilotons of TNT. The great mystery is why the universe is made almost entirely of matter. The Big Bang should have produced equal amounts of both, yet antimatter is vanishingly rare - this asymmetry remains unexplained.