Fig. 1: Photograph of Dirac and its negative: Dirac-AntiDirac.

Everything which surrounds us is made of matter. But what is antimatter? How did this concept come about?
To answer these questions we should go back in time to the 1930s.
In 1928 the British physicist Paul Dirac formulated a theory  for the motion of electrons in the presence of electric and magnetic fields; the theory included quantum effects and also relativistic effects.
This theory explained extremely well the experimental results and it led to a surprising prediction.
The electron should have had a corresponding "antiparticle" with the same mass and opposite electric charge.
Dirac's theory had experimental confirmation in 1932 with the discovery of the positron.

Fig. 2: Artistic representation of one hydrogen atom and one of anti-hydrogen. (Credit: INFN-Notizie N.13 Febrary 2003 "A caccia di antimateria: l'esperimento ATHENA" )

We now know that each particle with half integer spin has a corresponding antiparticle. While the mass of a particle and that of the corresponding antiparticle are exactly the same, other properties are equal but have the opposite sign. For instance, the antiproton has the same mass as the proton but the electric charge is opposite (the charge of the proton is positive, that of the antiproton negative).

Also to electrically neutral particles, like the neutrons, there are corresponding antiparticles. They have equal properties, some of which with the opposite sign (like the magnetic moment)

When matter and antimatter particles collide they annihilate into energy,  radiation.