Math Calculation

Two stages to that:

1. Many calculations in mathematics yield two answers. A simple example is the square root of 4: it’s 2, but it’s also -2, since -2 by -2 is also 4.
   
   Usually, when you do physics, you simply discard one of the answers. Length and weight cannot be negative, so you only deal with the positive number.
   
   In the 1920s, Paul Dirac got one of those “double” answers, but refused to discard the one that didn’t seem to correspond to anything physical. He just said that a positive electron fell out of his calculations, so it ought to be there to be discovered.
2. When playing around with particle accelerators a few decades later, experimentalists found something that looked exactly like an electron, but with a positive charge. Dirac had been right. In the next few years, the whole zoo of antimatter particles were found.