Helium is a chemical element, number 2 on the periodic table. It is the second most common element in the universe after hydrogen. Helium is the typical result of hydrogen fusion inside stars. Helium is highly non-reactive, being the first of the noble gases.
Helium was a typical component for a class T nebula along with hydrogen and argon. (VOY: "Tinker Tenor Doctor Spy")
Liquid helium was used for the engines of Enterprise NX-01. (ENT: "Silent Enemy")
The species that docked with the Automated repair station before Enterprise in 2152 lived in a liquid helium atmosphere, with a temperature of −270 °C. (ENT: "Dead Stop")
Helium fusion enhancement was at the center of Doctor Timicin's studies to re-vitalize the Kaelon sun. (TNG: "Half a Life")
An anomalous rogue comet investigated by the USS Enterprise-D in 2370 had an outer layer of gaseous hydrogen and helium. (TNG: "Masks")
The destruction of the Caretaker's array by the USS Voyager in 2371 left vapor consisting of hydrogen, helium, mercury, and argon. (VOY: "The Voyager Conspiracy")
The nucleogenic cloud being was composed mostly of hydrogen, helium, and hydroxyl radicals. (VOY: "The Cloud")
Trajis Lo-Tarik, a hallucination created by Seven of Nine while Voyager was traversing a nebula in 2374, offered liquid helium in trade for a microfusion chamber. (VOY: "One")