This isn't really a mystery. Mostly annihilating matter/antimatter becomes neutrinos or photons. The neutrinos became the cosmic neutrino background, which we h

This isn't really a mystery. Mostly annihilating matter/antimatter becomes neutrinos or photons. The neutrinos became the cosmic neutrino background, which we have no means of detecting. The CMB wasn't weak when it first decoupled from matter. You're looking now at something that has been red-shifted by 13 billion years of the expansion of space until it's in microwave wavelengths now. When it was first created, it was energetic enough to ionize all matter in the universe for nearly 400,000 years before it finally decoupled.