Antimatter is the opposite of normal matter. More specifically, the sub-atomic particles of antimatter have properties opposite those of normal matter. The electrical charge of those particles is reversed. Antimatter was created along with matter after the Big Bang, but antimatter is rare in today's universe.
In the heart of an atom, called the nucleus, are protons (which have a positive electrical charge) and neutrons (which have a neutral charge). Electrons, which generally have a negative charge, occupy orbits around the nucleus. The orbits can change depending on how "excited" the electrons are (meaning how much energy they have.)
These antimatter particles (which are called "antiparticles") have been generated and studied at huge particle accelerators such as the Large Hadron Collider operated by CERN (the European Organization for Nuclear Research), NASA stated.