Antimatter is the general name given to a category of particles that share the same properties as other forms of matter, only with a reversed charge. For example, the antimatter particle called a positron shares all the properties of an electron, but with a positive charge instead of a negative one.
Discovery of antimatter
The idea of an anti-particle was first developed by the physicist Paul Dirac in the late 1920s. Combining the emerging field of quantum mechanics with Albert Einstein's work on relativity, he revealed how particles behave at different speeds. Interpreting the consequences of his equations, Dirac suggested particles with the same mass and spin as electrons could theoretically exist, only with an opposite charge.
The following decade, tracks of particles left by cosmic rays inside a cloud chamber were considered to be the first sign that Dirac's anti-electrons existed in reality.
When antimatter particles meet with their matter equivalents, each particle decays into gamma radiation.
This transformation of energy has made antimatter the perfect fuel for everything from engines to weapons in science fiction. While it's a natural product of decay in radiating materials such as potassium, and can be generated using particle colliders, collecting enough in one place to serve as a source of power is challenging. For CERN to amass a gram of the material at its current rate of generation, it would take about 100 billion years.
Why is antimatter so rare?
So far there is nothing in physics that makes matter special. Both types of particle should exist in equal amounts, but why we don't see this remains one of the biggest mysteries in physics.
Since both forms of particles annihilate each other and leave only high energy radiation, it's also a mystery as to why we have particles of a particular variety at all.