Hydrogen makes up most of the observable universe by mass.
Hydrogen accounts for roughly 74 percent of the ordinary matter in the universe by mass, with helium contributing most of the remainder. It is the simplest chemical element, consisting of one proton and one electron in its most common form.
Most hydrogen formed during Big Bang nucleosynthesis, within the first minutes after the universe began expanding. Stars then use hydrogen as their primary nuclear fuel, fusing it into helium in their cores. This process releases the energy that makes stars shine.
Hydrogen is also the most abundant element in the Sun and in gas between stars. Earth contains far less free hydrogen than the universe overall because much of its hydrogen is bound in water and organic compounds.