Antimatter
Many scientists think that in the first few moments after the Big Bang, which created the universe a very long time ago, both matter and antimatter mixed together. If the Big Bang made equal amounts of matter and antimatter, then the two would annihilate and become energy. After a long time, there would be no matter and no antimatter left, just energy. But our universe today looks like it is almost all matter and hardly any antimatter. Physicists do not yet know for sure that equal amounts of matter and antimatter were created, and because of this, they are also wondering where the antimatter went, and if any was left over from the beginning of the universe.