Antimatter is material composed of elementary particles of equal mass but opposite electrical charge. The antiparticle of the electron is called the positron and that of the proton is called the antiproton. The antineutron has no electric charge, yet it is distinct from the neutron.
When a particle encounters its antiparticle, annihilation results and the two particles transform into a pair of high energy photons. Conversely, antimatter particles can be generated in high energy nuclear events, such as collisions inside particle accelerators. Antimatter needs to be kept confined in a magnetic field in order to keep it stable for any duration of time since it will annihilate as soon as it encounters matter.
The term CPT symmetry means that laws of physics remain invariant if electric charges are reversed, signs of coordinates reversed, so that x, y, z becomes -x, -y, -z and the direction of the arrow of time is reversed. Thus, antimatter could be said to behave like matter where charge, direction and time were reversed. Reversing only charge and parity is not sufficient since nuclear weak interactions show differences between matter and antimatter.
It is speculated that equal amounts of matter and antimatter were present immediately after the big bang. There is no evidence for large amounts of antimatter existing in the visible universe. If antimatter were concentrated in certain areas of space, annihilation reactions would be expected to occur between it and the matter surrounding it. No such thing is observed, at least not to a significant degree. Violations of CP (charge-parity) symmetry are invoked as justifications for the preponderance of matter in the observable universe.
Chemical properties of antimatter cannot in practice be observed because antimatter needs to be kept as plasma trapped in magnetic fields in order to prevent it from coming in contact with matter. Also, it is unknown whether gravity between matter and antimatter is attractive or repulsive. This is hard to determine because the force of EM is so much greater than that of gravity in the experimental situation.
The subject is more complex than we have space to adequately cover. The reader may look up antimatter and CPT and CP violation on the Internet for much more material.