Introduction:
Antimatter and matter could be defined as specular elements able to annihilate each other. The release energy by annihilation is so powerful that 0.5g of antimatter and matter would be enough to generate an amount of energy equal to the first nuclear bomb. Since the high amount of energy that could be generated by the use of antimatter many nations are investing large founds to deepen its nature.
The matter (which is only the 4% of the whole universe, ~68% is represented by dark energy and ~27% by dark matter) is made of atoms which in turn are made by particles such as quarks (like protons and neutrons) and leptons (like electrons). These particles are so small that one of them is to an atom as an atom is to our body.
12 main particles (6 quarks and 6 leptons) are defined as elementary particles since they can be arranged in different ways to make a wide range of particles. The elementary particles interact with each other through 4 fundamental forces: the strong and weak forces acting at subatomic distances and ruling nuclear interactions. and the gravitational and electromagnetic forces, which govern long range interaction.
The interactions of elementary particles through the weak, the strong and the electromagnetic forces are described by the theory of the Standard Model. The Standard Model, in order to work, needs elementary particles with the same characteristic of those belonging to the ordinary matter but with opposite charges. These are called elementary antiparticles and are considered the bricks of antimatter as the elementary particles are considered the bricks of matter. In fact also the elementary antiparticles can interact with each other making what are called anti protons or anti neutrons which in turn make the antimatter. Thus when an anti proton (with negative charge) collides with a proton (with positive charge) they annihilate themselves, releasing energy according to the well known law E = mc 2. The amount of mass (m) is (almost) depleted by the annihilation, so this means that it is converted (almost) entirely in energy (E) and a high amount is released with respect to the total mass involved in the event.
How it was discovered:
It was firstly theorized by Dirac in 1930 who was trying to reconcile quantum mechanics and general relativity. He noticed and show by his equations that particles may have a lower energy state than the one they have in the resting state, whence an anomalous opposite-energy quantum states. The first big clue was presented by Carl Anderson who discovered the positron in 1932. Anderson was investigating cosmic rays when he detected unexpected particle tracks in his cloud chamber photographs (particles detector).
This track was similar to the one generated by an electron but specular: the track was bent to the left instead of right. He interpreted, correctly that it was the track generated by an electron with opposite electrical charge, further called positron.
Finally the antiproton was discovered in 1955 by Owen Chamberlain and Emilio Segrè using a particles collider (Bevatron, placed in California) colliding a proton with a nucleon, the atomic nucleus of a proton.
How it is studied:
This experiment lays the foundation for the further experiments into particles and antiparticles by means of particle accelerator. Nowadays the most important particle accelerator is the Large Hadron Collider (LHC), a 27 km ring located in Geneva which aims at accelerating particles almost at the speed of light in order to increase the energy of impacts and thus the resulting particles swarm to study. Always in Geneva is located the Antiproton Decelerator (AD), a small ring (270 meters) used to decelerate particles still for studying purpose. The AD is the only place on earth where is possible to study the antihydrogen, an antiatom composed by one antiproton and one positron.
The unsolved mystery:
What remains unclear to physicists is why exist antimatter and matter and why in the universe matter prevails to antimatter. Current theories assess that during the big bang and the following universe expansion there was a displacement between matter and antimatter. Matter and antimatter started to annihilate but the amount of matter was one billionth more than antimatter. That one billionth is just the resulting survived matter that we see in the universe otherwise the universe would be made of light. Unfortunately we still do not know what tipped the balance in favor of matter.