Antimatter is made of antiparticles, which mirror ordinary particles in mass but carry opposite electric charge and other quantum properties. The antiparticle of the electron, called the positron, has the same mass as an electron but a positive charge instead of negative.
When a particle meets its antiparticle, the two annihilate each other completely, converting their entire mass into energy according to Einstein’s equation E equals mc squared. This makes antimatter both extraordinarily energetic and extraordinarily hard to store, since it destroys itself on contact with any ordinary container.
One of the biggest open puzzles in cosmology is why the universe is made almost entirely of matter rather than a fifty fifty mix of matter and antimatter. The Big Bang should have created equal amounts of both, and something in the early universe tipped the balance in favor of matter, a mystery physicists are still chasing.