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- Early in the Universe when temperatures were incredibly high, isotopes of hydrogen, helium, and lithium were synthesized in abundance. Hydrogen and helium are still incredibly abundant and make up nearly all of the mass in the Universe, though there is only about a third of the lithium-7 that we should see. There are a wide variety of explanations for why this might have happened, including some hypotheses involving hypothetical bosons known as axions, and others believe it is trapped in the core of stars, which our current telescopes and instruments can’t detect. However, there are currently no clear front running theories to explain this absence of lithium in the Universe.