As the same word says, the antimatter it is the opposite of matter, that is: a matter whose elementary particles have an electric charge opposite to normal. Thus, in an antimatter atom we find instead of protons (positive), antiprotons (negative) and, instead of electrons (negative), antielectrons or positrons (positive).
When a particle and an anti-particle come into contact, the phenomenon of annihilation occurs, that is, the transformation of matter into energy. Antimatter, theoretically foreseen by the physicists of the 30s, has been produced in laboratories since the mid-50s, thanks to the powerful particle accelerators.
According to a cosmological theory, in the Universe there are equal amounts of matter and antimatter confined, obviously, in regions far apart. However, at the meeting points, great annihilation phenomena would occur. Gamma rays, which are observed as background radiation of the Universe, are interpreted by some as the secondary product of this annihilation.
According to another theory, however, matter and antimatter existed equally in the origin of the Universe, but with a slight surplus of the first over the second. Therefore, antimatter would have been totally destroyed by annihilation and the current Universe would be constituted by the residue of surviving matter.
In the current state of physical knowledge it is impossible to determine, through astronomical observations at a distance, if a distant galaxy is made of matter or antimatter, because both produce identical electromagnetic emissions.
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