Antimatter is the opposite of matter. It has the same mass but with an opposing electrical charge. One theory about why antimatter exists was developed by John Wheeler and Nobel laureate Richard Feynman based on the idea that physical systems should be time-reversible. For example, the orbits of our solar system, if played backwards, should still obey all the same rules as when they are played forwards. This led to the idea that antimatter is just ordinary matter going backwards in time, which would explain why antiparticles have an opposite charge, since if an electron is repelled while going forwards in time, then backwards in time this becomes attraction. This also explains why matter and antimatter annihilate. This isn’t a circumstance of two particles crashing into and destroying each other; it is the same particle suddenly stopping and going back in time. In a vacuum, where a pair of virtual particles are produced and then annihilated, this is actually just one particle going in an endless loop, forwards in time, then backwards, then forwards, and so on.

While the accuracy of this theory is still up for debate, treating antimatter as matter going backwards in time mathematically comes up with identical solutions to other, more conventional theories. When it was first theorized, John Wheeler said that perhaps it answered the question of why all electrons in the universe have identical properties, a question so obvious that it is generally ignored. He suggested that it was just one electron, constantly darting all over the universe, from the Big Bang to the end of time and back again, continuing an uncountable number of times. Even though this idea involves backwards time travel, it can’t be used to send any information back in time, since the mathematics of the model simply doesn’t allow it. You cannot move a piece of antimatter to affect the past, since in moving it you only affect the past of the antimatter itself, that is, your future.