Final answer:
Nuclear fusion reactions in the early, hot universe determined the amount of Helium and Deuterium, which are influenced by the universe's density.
Step-by-step explanation:
The amount of Helium and Deuterium in the expanding universe is determined by nuclear fusion reactions. In the hot, early universe, energy of photons produced material particles when they collided. As the universe expanded and cooled, fusion reactions took over and produced deuterium, helium, and lithium nuclei. The deuterium abundance in the current universe helps estimate the amount of ordinary matter, which is only about 5% of the critical density. The rate of these fusion reactions and the resulting production of helium and deuterium were influenced by the universe's density at that time.