According to theories.. In the early universe.. There were particles and anti-particles in equal number.. But there seems to be one extra particle against the number of anti-particles.. As the theories say... But how did that extra particle come from.. Because from the photons equal number of particles and anti particles were produced?
The common hypothesis supposes that there is a slight asymetry in a transformation (i.e., slightly more likely in one direction than the other), which is called the "violating CP symmetry" (occuring in the weak interaction). See wikipedia "baryon asymmetry" and "CP violation".