The temperature of the universe just after the Big Bang was extremely high. In the earliest moments of the universe, scientists believe that the temperature reached an astonishing value of about 10^32 Kelvin (K) or even higher. At such high temperatures, the fundamental forces of nature were unified, and the universe was in a state known as a "hot, dense plasma."
As the universe expanded and cooled down, it underwent a phase transition called cosmic inflation, during which the temperature dropped dramatically. After inflation, the universe continued to cool further as it expanded. Within the first few minutes after the Big Bang, the temperature dropped to around a billion (10^9) Kelvin, and the formation of atomic nuclei, known as nucleosynthesis, occurred. This process produced the lightest elements like hydrogen and helium.
It is important to note that these temperature values are theoretical estimates based on our current understanding of the early universe, as they are derived from models and extrapolations from particle physics and cosmology. The precise details of the early moments of the universe are still an active area of research, and further observational and theoretical advancements may refine our understanding in the future.