For experts: Anti-Particles

For every matter particle there exists a particle which has exactly the same mass but opposite charge-like quantum numbers - these are inner properties of the particles, for example the electric charge or the color charge of the quarks. These particles are called the corresponding anti-particles.

In 1928, the existence of anti-matter was predicted by Paul Dirac. In 1932, the anti-electron (positron) was experimentally detected for the first time in cosmic rays. Today we know that for every particle there exists an anti-particle, for example anti-quarks, anti-neutrinos or anti-electrons.

In short

• Usually a cross bar above the particle symbol is used to mark an anti-particle.  
• An anti-particle can never be created alone. There is always a particle antiparticle pair created
• If a matter particle meets its anti-particle, they annihilate each other (pair annihilation) and energy is produced
• Electron-positron annihilation is used in medicine, for example, to make tumors visible (positron emission tomography, PET)
• There are particles that are their own anti-particles, e.g. the photon or the neutral J/Psi (a bound state of a charm quark and an anti-charm quark)

In the early formation phase of our universe - fractions of a second after the big bang - creation and annihilation of particles were in an equilibrium. That is, particles and anti-particle pairs were created and annihilated again. The universe expanded and therefore cooled down. After a few seconds the universe cooled down that far the energy was no longer sufficient to create new pairs.
Thereby all matter and anti-matter should have annihilated. However, we observe today a universe made only of normal matter, so the annihilation of particles and anti-particles could not have been complete. Scientists think that there were decays of particles that created slightly more matter than antimatter. From the small excess of matter - about 1 particle of matter for every 1,000,000,000 particles that were annihilated - all the matter visible in the universe today was formed.

To this day, it is not understood where the matter-antimatter asymmetry in the universe comes from. This is the subject of current research at CERN, especially at the LHCb experiment.