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).
A bit is the result of nuclear fusion of hydrogen in stars, but most helium 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 value which observations/analysis confirm and refine. Methods of determining it 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 and of opposite the typical spin. It can remain in this state for a couple of hours and on relaxing, produces an emission line of about 80830 angstroms.