Antimatter forms in nuclear reactions. In the sun and other stars, lots of antimatter is formed and destroyed. Antimatter can be made at places such as CERN by crashing high energy beams of particles into each other (which is how many forms of antimatter were discovered, as well as heavier particles of matter). Some nuclear reactors produce antimatter but it is very small, and usually unoticible. The nuclear reactions that produce antimatter involve heavy unstable particles decaying to form stable particles, such as quarks and electrons. When such a heavy particle decays several things happen. The first thing that happens is that charge must be conserved; meaning that if an object with no charge decays to form an electron with a charge of -1 another particle (or in some cases particles) with a charge (or total charge) of +1 must also be created. This is the antimatter, as it has the opposite charge to the particle created (the characteristic of antimatter), if the example of a particle with charge -1 was an electron a positron (or anti-electron) would also be made. However these reactons usually give off energy as well, as the law of conservation of energy says energy cannot be lost or created. And as the famous equation E=mc^2 says heavier particles have more energy because of larger masses therefore the two smaller particles made have less energy than the bigger particle and the extra energy is released as an electromagnetic wave (or a light wave). The energy released in these and other similar reactions is why stars give off so much light. I hope this explained what you wanted to know...