Solving the missing baryon problem: A review of observational and theoretical advances

The missing baryon problem represents a longstanding challenge in cosmology, highlighting a discrepancy between the amount of baryonic matter predicted by cosmological models and the amount directly observed in the universe. While observations of the cosmic microwave background and Big Bang nucleosynthesis accurately constrain the baryon density of the early universe, only a fraction of this baryonic matter is accounted for in stars, galaxies, and hot gas within galaxy clusters today. Recent advances suggest that much of the missing baryonic matter resides in the warm–hot intergalactic medium (WHIM), a diffuse, filamentary gas with temperatures of \(10^5\)–\(10^7\) K. Detecting the WHIM has been challenging due to its low density and weak emissions. However, breakthroughs in observational techniques, such as X-ray and UV spectroscopy, along with cosmological simulations, have provided compelling evidence for its presence. This review synthesizes the latest theoretical and observational efforts to locate the missing baryons, emphasizing the role of the WHIM, novel detection strategies, and their implications for understanding large-scale cosmic structure and galaxy formation. Future missions promise to refine these findings, bringing us closer to resolving this fundamental issue in astrophysics.