Cosmological lithium problem

In astronomy, the lithium problem or lithium discrepancy refers to the discrepancy between the primordial abundance of lithium as inferred from observations of metal-poor (Population II) halo stars in our galaxy and the amount that should theoretically exist due to Big Bang nucleosynthesis (BBN). The Big Bang nucleosynthesis and WMAP constitute cosmic baryon density predictions of the cosmic microwave background (CMB); namely, the most widely accepted models of the Big Bang suggest that three times as much primordial lithium, in particular lithium-7, should exist.

Source: Wikipedia — Cosmological lithium problem (CC BY-SA 4.0)