Hydrogen, for sure. It is the simplest of elements, with only one proton (and maybe a neutron or two) in the nucleus. Jamming protons together to create heavier elements takes a star (like our sun, turning hydrogen into helium) and even hotter stars, novas, or supernovas to make heavier elements. Most of the matter in the universe has never fallen into a star.
We can know this because we can detect and identify elements throughout the universe through astronomy as they reflect, radiate, and absorb energy in unique ways.