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.
In practice, antimatter is not just a theoretical curiosity, hospitals rely on it every day. PET scanners, used to image tumors and study brain activity, work because a radioactive tracer emits positrons inside the body, and detecting the pair of gamma rays released when those positrons annihilate with nearby electrons lets doctors map exactly where the tracer went. Producing antimatter in bulk is far harder than detecting the trace amounts made in a scanner. CERN’s Antiproton Decelerator can only make and trap tiny quantities of antihydrogen at a time, cooled and held in place by magnetic fields since it would destroy itself instantly on touching any physical container. At current production rates, making even a single gram of antimatter would take longer than the age of the universe and cost far more than any nation’s budget, which is one reason antimatter propulsion, a staple of science fiction, remains firmly out of reach for real spacecraft.