According to Big Bang Theory, matter and antimatter should have been created in equal amounts.
It's currently estimated, however, that :
“The Standard Model prediction for the ratio of baryons over antibaryons deviates from the astrophysical observations by eight orders of magnitude.”
There could be areas of the universe where antimatter dominates - but no observations have so far identified regions in space where antimatter and matter encounter each other, catastrophic zones which would be easily detectable due to the radiation that would be produced.
To compound the mystery :
“Theories that explain this apparent asymmetry violate other measurements.”
Source:. [link needs clarifying]
 Where is all the antimatter? New Scientist, April 2009
“One would expect the Big Bang to produce equal amounts of matter and antimatter, and, since the two annihilate one another on contact, this should have led to a universe with no particles, filled only with radiation.
This problem can be solved if there exists some process that favours matter over antimatter, leading to the excess that we see today.
Alternative explanations include the possibility that there are regions of the universe made of antimatter – which is thought to be unlikely since any overlap with matter regions would produce easily detectable radiation – and the suggestion that antimatter also exhibits gravitational repulsion, which would keep such regions separate.”
New research conducted at CERN has attempted to find fundamental differences between protons and anti-protons. Without success. Finding asymmetries could explain how one form (matter) is dominant in the universe.
See: Riddle of matter remains unsolved: Proton and antiproton share fundamental properties Press release, Johannes Gutenberg-Universität Mainz, Oct. 2017