What is the most abundant element in the known universe?

The most abundant element in the known universe is hydrogen. It is the lightest chemical element, has atomic number 1, and makes up roughly 75% of ordinary matter by mass; helium is the second-most abundant.

Hydrogen formed in enormous quantities during the first stages of the Big Bang. As the early universe cooled, protons captured electrons to form neutral hydrogen atoms. Today, hydrogen remains the primary fuel of stars, where nuclear fusion converts hydrogen into helium and releases the energy that makes stars shine.

In space, hydrogen may be found as neutral atomic hydrogen, ionized hydrogen in hot plasma, or molecular hydrogen made of two hydrogen atoms. On Earth, it is less common as a free gas because it readily combines with oxygen and other elements. A common mix-up is answering helium: helium is abundant in stars and the universe, but hydrogen is the clear first-place element.