All matter is made of atoms, and atoms are made in part of electrically charged particles called protons and electrons. In normal matter, electrons always have a negative electrical charge, and protons always have a positive charge. But physicists have found that, for every type of particle, there’s an antiparticle — with the opposite electric charge. Atoms of antimatter would have an anti-nucleus — made up of anti-protons and anti-neutrons — surrounded by a cloud of anti-electrons, known as positrons.
Just by looking, you couldn’t tell antimatter from ordinary matter. But when a particle meets its antiparticle, the two will totally annihilate each other — they will release enormous amounts of energy. Suppose you shook hands with an anti-you. The result would be an enormous explosion – pretty tough to miss. A less dangerous way to detect antiparticles is to let them move through a magnetic field. Since they have the opposite charges of regular particles, the magnetic field curves them in the opposite direction.
This is how the first anti-particle, the anti-electron or positron, was discovered in the spring of 1932.
Fortunately, the Earth — and the whole observable universe — seem to be mostly matter, not an explosive mixture of matter and anti-matter. At every day energies, though, the laws of physics are the same for matter and anti-matter, so it’s a big mystery that our universe favors one over the other. But at very high energies, physicists see subtle differences that can distinguish matter and anti-matter, and it’s believed — but not fully understood — that these are how the hot early universe decided to make more matter.
Dr. Sterl Phinney, Professor of Theoretical Astrophysics at Caltech, adds:
There are bits of antimatter being created around (and even inside) you all the time, but they are rapidly destroyed by annihilation with the much more abundant regular matter. Cosmic rays and high-energy photons from the sun and more distant astronomical objects strike the earth and its atmosphere and create sprays of matter and antimatter (including the antielectron in Carl Anderson’s cloud chamber). The high energy particles created in particle accelerators do the same thing: the famous accelerators at Stanford and at CERN in Geneva collect the antiparticles, accelerate them and then run them head on into particles. The resulting annihilations create very high energies and new kinds of particles. Some radioactive elements made in reactors and bombs (but not any of the ones naturally present in rocks) emit antielectrons during their decay. Far more antimatter is being made out in space. Supernovae create huge amounts of radioactive matter which creates antielectrons as it decays. We see the radiation from these antielectrons annihilating in our Galaxy. Even more antielectrons are created by the dense radiation near accreting black holes. The jets we see coming out of these are probably (the proof isn’t airtight yet) composed of an equal mixture of electrons and antielectrons.
Only neutral particles with no internal structure (like the photon) are their own antiparticles. Neutral particles with internal structure (like the neutron) are not their own antiparticles. One can imagine that this is because the sign of charge of all the whirling charges inside them are reversed. This explains why the antineutron has the opposite magnetic moment to the neutron. And particles (like the electron) whose spin is odd multiples of half of Planck’s constant have the opposite parity (handedness) to their antiparticles as well as the opposite charge.