Final answer:
In the immediate aftermath of the Big Bang, the universe was extraordinarily hot, housing a plasma of elementary particles and atomic nuclei that eventually cooled down to enable the formation of atoms, stars, and galaxies.
Step-by-step explanation:
In the earliest times of the first few minutes after the beginning of the expansion of the universe, it was so hot that the universe contained a plasma of elementary particles such as neutrons, protons, electrons, and neutrinos, as well as the atomic nuclei like deuterium, helium, and a trace of lithium which formed when the universe was 3-4 minutes old. The temperature was so extreme, at about 100 billion K just 0.01 second after the Big Bang, and even after 3 minutes, it still remained at about 1 billion K, a factor of 70 hotter than the interior of the Sun. As the universe continued to cool and expand, these hot conditions fostered the early phase of the universe until it cooled sufficiently to allow for the formation of hydrogen atoms, leading to the universe becoming transparent to light and setting the stage for the formation of stars and galaxies in the subsequent millions of years.