Editing Deuterium (section)

===Natural Abundance===
Deuterium occurs in trace amounts naturally as deuterium gas, written ²H2 or D2, but most natural occurrence in the universe is bonded with a typical ¹H atom, a gas called hydrogen deuteride (HD or ¹H²H).

The existence of deuterium on Earth, elsewhere in the solar system (as confirmed by planetary probes), and in the spectra of stars, is an important datum in cosmology. Stellar fusion destroys deuterium, and there are no known natural processes (for example, see the rare cluster decay), other than the Big Bang nucleosynthesis, which might have produced deuterium at anything close to the observed natural abundance of deuterium. This abundance seems to be a very similar fraction of hydrogen, wherever hydrogen is found. Thus, the existence of deuterium is one of the arguments in favor of the Big Bang theory over the steady state theory of the universe. It is estimated that the abundances of deuterium have not evolved significantly since their production about 13.7 billion years ago.