What is Antimatter?
A combination of quantum mechanics and relativity allows us to examine subatomic processes in a new light. Symmetry is very important to physical theories. For example, conservation of momemtum is required for symmetry in time. Thus, the existence of a type of `opposite' matter was hypothesized soon after the development of quantum physics. "Opposite" matter is called antimatter.
What is it made of?
Particles of antimatter has the same mass and characteristics of regular matter, but opposite in charge. When matter and antimatter come in contact they are both instantaneously converted into pure energy, in the form of photons.
Where does it come from?
Antimatter is produced all the time by the collision of high energy photons, a process called pair production, where an electron and its antimatter twin (the positron) are created from energy (E=mc2).