Definition
Antimatter is a special kind of matter made of particles that match ordinary matter in mass and other physical traits, but have the opposite sign for properties like electric charge. When matter and antimatter meet, both particles vanish, and all of their mass turns into a huge burst of energy.
Flipped particles, like a world in a mirror
Step into a funhouse mirror room and you see someone who looks just like you, but flipped left to right. Nature has something similar: twin particles that look exactly like their partners, except that the signs of a few properties are reversed. Science calls this antimatter.
Everything we meet in daily life is made of tiny atoms. At the center of an atom sits a proton with a positive (+) charge, and a negatively (-) charged electron circles around it. An atom made of antimatter has the same structure, mirrored. A positively (+) charged "positron" circles a negatively (-) charged antiproton.
Amazingly, their weight (mass) and size match down to the last decimal place. Only the signs of a few properties, charge included, are flipped. That is why even the neutron, which has no charge, has its own antiparticle, the antineutron.
Meet matter, and vanish into pure energy
Just as plus and minus cancel cleanly to zero, matter and antimatter set off the most dramatic reaction there is when they meet. The moment two particles collide, they disappear without leaving any shape behind and give off a huge flash of light and heat. This striking event is called annihilation.
When you burn wood or coal, only a tiny part of the material turns into heat. Even nuclear power plants turn less than 1 percent of the mass into energy. But when matter meets antimatter, all of the mass of both turns into energy, following the principle Einstein uncovered. An electron and a positron turn straight into light called gamma rays. A proton and an antiproton pass through several other particles and end up scattered as gamma rays and particles such as neutrinos.
Just 1 gram of antimatter would release enough energy to launch a giant rocket into space. On Earth, any antimatter that forms hits the matter around it right away and turns into light in an instant. That is why we hardly ever see it.
To be more precise: the mystery of the missing antimatter
It sounds like science fiction, but antimatter is real. Particle accelerator labs make it on purpose and study it. Cosmic rays, which are high-energy particles flying in from space, also create positrons for a split second when they hit Earth's atmosphere.
To be more precise, antimatter is a key to the origin of the universe for physicists. About 13.8 billion years ago, when the Big Bang started the universe, matter and antimatter should have been born in exactly equal amounts. By theory, they should have collided and wiped each other out, leaving a universe full of nothing but light.
But, almost like a miracle, ordinary matter ended up slightly ahead, by about 1 part in a billion. That small handful of leftover matter gathered into stars, planets, and the universe we live in today. Scientists now use giant particle colliders to work out why matter won out over antimatter.
Common misconceptions
Antimatter is anti-gravity material that floats up into the sky because gravity pushes it the other way.
Antimatter has positive (+) mass, just like ordinary matter, so it falls down. Only its charge is opposite. Gravity works on it as usual.
Where you meet it
In particle accelerator labs, scientists smash particles together at close to the speed of light to make antimatter and study it.
When high-energy particles from space hit Earth's atmosphere, antimatter such as positrons forms naturally.
In one sentence
Antimatter is matter's twin with only the charge reversed, and when it meets matter, all of the mass turns into a huge amount of energy.