 The inputs to nucleosynthesis were baryons, protons, and neutrons. Baryogenesis is the process that created these baryons. Quantum chromodynamics shows how photons can create matter-antimatter pairs, and how matter-antimatter pairs can create photons. If we go back in time to when the universe was only 20 trillionths of a second old, its temperature was over a thousand trillion degrees, and photon energy was around 100 gigaelectron volts. At that level, no baryonic matter could survive, and all of space would have been filled with pure radiation. Then, when the temperature of the universe cooled to around 1.74 trillion degrees, 33 microseconds into its expansion, created quarks and anti-quarks could survive. These quarks were not confined within baryons as they are today. Instead, they formed a sea of free quarks, a quark soup, a quark plasma. During this period of quark-anti-quark production, the number of quarks, anti-quarks, and photons were in thermal equilibrium, and therefore present in equal numbers. But the universe today has almost no antimatter at all. Now, suppose nature had a very tiny tilt in favor of quarks over anti-quarks. For example, let's say that for every 800 million and three quarks, there were only 800 million anti-quarks. Then, when the universe cooled to the point that quark-anti-quark pairs were no longer being produced, all the existing quarks and anti-quarks would annihilate each other. In our example, only three quarks would remain, and these three quarks would be surrounded by 1.6 billion photons, the product of the annihilations. As the expansion continued and temperatures continued to cool, the free quarks were bound into protons and neutrons with the resulting baryon to photon ratio to our familiar 6 times 10 to the minus 10. Baryogenesis was over. Before the 1960s, elementary particle theory held that the laws of physics were exactly the same for matter and antimatter. Theorizing a process that selects one over the other to create a small imbalance was at odds with this equality premise. But in the 1960s, small differences in kaon decay showed that nature does treat them differently. But so far, no good explanation for how things happen during baryogenesis has been developed.