Why is hydrogen the most abundant element in the universe?

Hydrogen is the most abundant element in the universe because of the conditions present during the early stages of the big-bang theory of the universe's creation.1

According to this theory, the universe began approximately 13.8 billion years ago in a state that was extremely hot and dense. As the universe expanded and cooled, nuclear reactions occurred that established a well-defined initial chemical composition. During this period of primordial nucleosynthesis, neutrons and protons interacted to form deuterium, which then rapidly assembled into helium nuclei.12

Because there was a significant excess of protons relative to neutrons, not all of the protons were consumed in the formation of helium. The remaining protons—which are the nuclei of hydrogen—accounted for the vast majority of the matter by mass. Calculations based on this model predict that the primordial universe consisted of approximately 75 percent hydrogen and 25 percent helium by mass, a ratio that remains consistent with current astronomical observations of the universe's composition.21