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The missing baryon problem is the fact that current theory (the model of Big Bang nucleosynthesis and analysis of CMB anisotropies in light of Lambda-CDM model) indicates there was more baryonic matter in the early universe than has been accounted for in observed galaxies and intergalactic medium (IGM), these accounting for only roughly two thirds of what was expected, suggesting the existence of some unseen matter other than dark matter. Recent studies using the Sunyaev-Zel'dovich effect (SZ effect) indicate otherwise-unobserved high-density regions of the IGM that would account for it.