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What is the most common element in the universe?
Hydrogen makes up roughly three quarters of ordinary matter by mass, with helium most of the rest.
Hydrogen is the simplest atom, one proton with one electron, and it dominates the cosmos: roughly 73 to 75% of ordinary matter by mass and over 90% by number of atoms. Helium makes up most of the rest, at about 24 to 25% of the mass, and everything heavier combined is only a couple of percent. These proportions are measured from the light of stars and gas clouds and match predictions of Big Bang nucleosynthesis.
The hydrogen and most of the helium were produced in the first few minutes after the Big Bang, when the universe was hot enough for nuclear fusion. Once the universe cooled, fusion stopped, leaving a mix of about three parts hydrogen to one part helium by mass, plus traces of deuterium and lithium. Heavier elements such as carbon, oxygen and iron were not formed until later, inside stars and in explosions such as supernovae and neutron-star mergers.
On Earth the picture looks very different because the planet formed from heavy-element material. Oxygen, silicon, iron and magnesium dominate the planet by mass, and hydrogen is mostly bound into water. Hydrogen is also the fuel of stars like the Sun, which fuses about 600 million tonnes of it into helium every second. The question refers to ordinary matter, so dark matter and dark energy are not counted.