At present, there is no experimental evidence of any significant concentration of antimatter in our observable universe. In other words, the universe we live in consists almost entirely of matter. This phenomenon is puzzling to physicists, given the symmetry between matter and antimatter.
Many competing theories attempt to explain how such an asymmetry could have come about. One group of theories focuses on understanding how nature, at the particle level, might favour certain matter reactions in comparison to their antimatter counterparts. Such reactions have been observed and studied extensively in the laboratory, but we do not know whether they alone can explain the matter imbalance in the universe. Other theories propose that there are indeed regions of the universe composed primarily of antimatter (a so-called antiuniverse), but that these regions are widely separated from matter-dominated regions or are possibly outside of our visible universe. After all, there may be more to the universe than can be seen from Earth!