text: Antimatter (or, rarely, contra-terrene matter) is matter that is composed of the antiparticles of those that constitute normal matter. If a particle and its antiparticle come in contact with each other, the two annihilate, that is they are both converted from matter to energy in accordance with Einstein's equation e = mc2, giving rise to a brief flood of high energy photons (eg gamma rays) and possibly other particle/antiparticle pairs. Because c2 has a large value, the resulting photons and particles are endowed with a large amount of energy, ie, the difference between the rest energy of the products of the annihilation and the rest energy of the original particle/antiparticle pair.\n\nAntimatter is not found naturally on earth, except very briefly and in vanishingly small quantities (as the result of radioactive decay or cosmic rays). This is because antimatter which came to exist on earth outside the confines of a suitable physics laboratory, would almost instantly meet the ordinary matter that earth is made of, and be annihilated. Antiparticles, and some stable antimatter (such as antihydrogen), can be made in miniscule amounts, but not in enough quantity to do more than test a few of its theoretical properties.\n\nThere is considerable speculation both in science, and science fiction, as to why the Earth is almost entirely matter, and if other places are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. Possible processes by which it came about are explored in more detail under baryogenesis.