Helium (He) is the element with atomic number 2, symbol He. Its most common isotope has mass number 4, but mass number 3 is also stable. It is the second most common element, about 24% of the observable universe by mass (helium abundance).
Some helium is the result of nuclear fusion of hydrogen in stars, but most was formed in conditions shortly after the Big Bang. The initially-very-hot universe descended through temperatures that produced a particular ratio of neutrons and protons, which at a cooler temperature, combined into low-mass isotopes, including a very stable helium isotope (mass number 4), much of which remained as the temperature cooled further. Helium's primordial abundance, Yp ("Y" standing for a mass fraction of helium) is a subject of study. Theory of Big Bang nucleosynthesis produces a Yp value which observations/analysis confirm and refine. Methods of determining Yp include determining a ratio of changes in the abundance of helium and metals (to extrapolate backwards) and observing/analyzing very early HI regions at a time when metallicity was low.
Helium has a metastable state of excitation with one electron excited, with opposite the typical spin. An atom can remain in this state for a few hours during which it can absorb photons of wavelength 80830 angstroms. Such atoms can produce a metastable helium line not produced by helium solely in the ground state.