On the periodic table, hydrogen is the most abundant element in the universe by mass.
Hydrogen is the simplest element, with one proton and one electron in its most common form. It makes up roughly three-quarters of the ordinary matter in the universe by mass, although the exact proportion varies between cosmic environments and measurement models.
Stars are mostly hydrogen and helium. In stellar cores, nuclear fusion converts hydrogen into helium and releases the energy that makes stars shine. Over immense timescales, this process also creates heavier elements in stars and stellar explosions.
Hydrogen is the lightest element, but it is not always the most common element in every location. Earth’s atmosphere contains relatively little free hydrogen because the gas is light and can escape into space. Water and organic compounds contain hydrogen in chemically bound form.