Immediately after the Big Bang, which occurred around 13.8 billion years ago, the universe was dominated by unimaginably high temperatures and densities. However, after just a few seconds, it had cooled down enough for the first elements to form, primarily hydrogen and helium. These were still completely ionized at this point, as it took almost 380,000 years for the temperature in the universe to drop enough for neutral atoms to form through recombination with free electrons. This paved the way for the first chemical reactions.
Chemistry at the beginning: How molecular reactions influenced the formation of the first stars
