The place is England. The year is 1928. One of the founding theorists of quantum mechanics , Paul Dirac, is scratching his head because solutions to his equations have  _ unexpected results. For the solutions to make sense, he reasons, there must be a particle that has the mass of an electron but the opposite charge. At the time, such a thing was not known to exist.  Several years pass before American physicist Carl Anderson observes a "positive" electron, or positron that confirms Dirac's prediction.  Antimatter , as the name implies, can be described as the opposite of ordinary matter. Every particle in the universe has characteristics such as mass and charge. With antimatter, the mass remains constant, but the sign of the charge is reversed. All particles have an antimatter counterpart , even the chargeless neutron .  Unlike matter, antimatter is not common. Unless you're in the upper atmosphere, or inside a particle accelerator, you're not going to come across it. "Antimatter was not always so rare," Stephane Coutu, Penn State particle physicist says. There was a time when it was as prevalent as matter itself. "Right after the Big Bang ," Coutu explains, "we believe there must have been exactly the same amounts of matter and antimatter...and yet owing to some small asymmetry in the laws of particle interactions, all of the antimatter and most of the matter in the early universe was annihilated . We are left today with the resulting matter-dominated universe."  Science fiction is rife with tales of high-energy particle annihilation, and indeed, antimatter weapons have appeared in current bestselling novels. This is unrealistic, Coutu says. "[It] would be very impractical owing to very great difficulties in producing and maintaining significant amounts of antimatter."  Technology that uses the properties of antimatter is actually feasible  outside of science fiction, however. Positron emission tomography (PET) is a medical technique that can be used to detect cancer, measure blood flow and detect coronary artery disease. While antimatter may never be used as a bomb, it certainly has a positive future in life-saving medical diagnostic tools, the anti-weapon. The main idea of the passage is   _  .
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