Missing baryon problem

The missing baryon problem is the historical mismatch between cosmological predictions of ordinary matter and the smaller amount initially detected in galaxies and galaxy clusters. Surveys later found much of the unaccounted matter as diffuse, hot gas in the cosmic web.

Cosmic microwave background: The relic radiation from the early universe, whose measured fluctuations constrain cosmological parameters. Its baryon-density measurement supplies the predicted inventory against which nearby matter was compared.
Fritz Zwicky: A Swiss astronomer who used galaxy motions to infer unseen mass in the Coma Cluster. His cluster mass discrepancy concerned gravitating matter broadly, predating the specific baryon accounting problem.
X-ray astronomy: Astronomy that studies celestial objects through their emitted or absorbed X-rays. X-ray observations measure the hottest gas in galaxy groups and clusters.
Dark matter: Matter inferred from gravity that emits, absorbs, and reflects little or no light. The missing-baryon problem concerns ordinary matter, not the much larger dark-matter component.
Big Bang nucleosynthesis: The formation of light atomic nuclei during the universe’s first minutes. Primordial element abundances provide an independent estimate of the universe’s baryon density.
Cosmic baryon census: An accounting of ordinary matter across cosmic environments and observational tracers. The missing-baryon problem arose when the census fell short of the cosmological inventory.
Ultraviolet astronomy: Astronomy using ultraviolet radiation to study celestial sources and intervening matter. Ultraviolet spectra reveal absorption from ionized oxygen in diffuse intergalactic gas.
Missing mass problem: A broad discrepancy between gravitationally inferred mass and directly observed matter. This wider label often concerns dark matter, whereas the baryon problem tracks ordinary matter specifically.
Warm-hot intergalactic medium: Diffuse intergalactic gas with temperatures roughly between 100,000 and 10 million kelvin. This difficult-to-detect gas accounts for much of the baryonic matter absent from galaxies.
Lambda-CDM model: The standard cosmological model combining a cosmological constant with cold dark matter. Its early-universe parameters predict the total baryon budget used in the discrepancy.

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