The phenomenon you are referring to, where photons can spontaneously convert into particle-antiparticle pairs, is known as pair production. Pair production can occur in the presence of a sufficiently high-energy photon interacting with a nucleus or an atomic electric field. However, the specific conditions necessary for pair production depend on the energy of the photons involved.
Pair production can be described by Einstein's famous equation E = mc², which relates energy (E) to mass (m) through the speed of light (c). According to this equation, a photon can spontaneously transform into a particle-antiparticle pair if its energy is greater than or equal to the combined mass-energy of the resulting particles.
Regarding the slight excess of particles over antiparticles in the universe, this is indeed a fundamental puzzle in physics known as the baryon asymmetry problem. Antimatter, consisting of antiparticles with opposite charges to their corresponding particles, is expected to be produced in equal amounts as matter in the early universe. However, our observations indicate that there is significantly more matter than antimatter.
The exact reason for this imbalance is not yet fully understood and remains an active area of research. Several theories and mechanisms have been proposed to explain this phenomenon, such as violations of certain symmetries in particle interactions, known as CP (charge-parity) violation, or through processes occurring during the early stages of the universe, such as phase transitions.
Further studies are being conducted at particle accelerators and through astrophysical observations to investigate the properties of fundamental particles and their interactions, which may shed light on the origin of this matter-antimatter asymmetry and the "missing" antimatter in the universe.