What was determined at the end of the era of nucleosynthesis in the universe?

At the end of the era of nucleosynthesis, which took place during the first few minutes after the Big Bang, the universe underwent a critical phase where fundamental particles began to fuse together to create the lightest elements. This process mainly produced hydrogen, helium, and small amounts of lithium and beryllium. By the end of this era, roughly three minutes post-Big Bang, the temperature of the universe had dropped sufficiently to allow protons and neutrons to combine into these nuclei.

This event is significant because it set the foundational chemical composition of the universe, establishing the ratios of these light elements that we observe today. While the universe continued to evolve and more complex structures formed later, the fundamental elements created during nucleosynthesis were crucial for the subsequent formation of stars, galaxies, and all heavier elements through stellar nucleosynthesis.

The other options refer to concepts that either occur later in cosmic evolution, such as the formation of stars or the stabilization of temperature, or they suggest scenarios, like a universe filled with antimatter, which do not accurately reflect the outcomes of the nucleosynthesis phase. Thus, the primary takeaway from this era is indeed the establishment of the basic chemical composition of the universe.