The two most common elements in the universe are also the two smallest ones: hydrogen and helium. Hydrogen is by far the most common element in the universe because it was the first element in the universe to be created. The Big Bang caused all of the hydrogen in the universe to be formed at the beginning. This hydrogen was then the basis for all heavier elements to be formed through fusion reactions. Since helium is formed by the fusion of two hydrogen atoms, it is the second most common element in the universe. The heavier elements like iron are much less common in the universe since they require more fusion reactions to occur.