The primordial baryon density (Omega_b, the fraction of the universe's critical density contributed by ordinary baryonic matter) inferred from Big Bang Nucleosynthesis observations of light element abundances in the intergalactic medium differs slightly but persistently from the value inferred from cosmic microwave background anisotropy measurements by Planck. The BBN-derived Omega_b tends to be marginally lower than the CMB-derived value, suggesting either systematic uncertainties in nuclear reaction rates, stellar abundance measurements, or a deeper inconsistency in the standard early-universe model (Cyburt et al. 2016; Planck Collaboration 2018).