A slight k-meson decay assymetry
Hi Jody
The Universe makes a distinction between matter and anti-matter in some weak force interactions, thus a very slight difference in baryon/anti-baryon numbers. The vast majority of the particles that condensed out of the initial radiation field were mutually annihilated - roughly a billionth survived. As the Universe expanded that billion to one particle-to-photon ratio declined in energy and the mass of the particles dominated by the time of the decoupling of light-and-matter after about 380,000 years.
No big mystery really.