During this stage of the Universe's life, there were equal amounts of antimatter and matter. When the two opposite particles met up, they would annihilate each other. This created more radiation that sparked even more particle births. By the first second of its life, the Universe had only matter. This was true on both the matter and the antimatter particles.
Particles decay into smaller particles because the larger particles are unstable.
They decayed into both particles and antiparticles but for each 100 million particles, there were only 99,999,999 antiparticles. Even though this was a tiny imbalance, it created all of the matter in the Universe. When all of the larger particles were unstable and started to decay, the smaller particles stayed the same. Even lightweight leptons and quarks decay too. There are six types of leptons and quarks.