For two decades roughly a third of the universe's ordinary matter was missing: big bang nucleosynthesis and the CMB fix the baryon density precisely, but summing everything observable at low redshift, stars, galaxies, cluster gas, and the detectable intergalactic medium, recovered only 60 to 70 percent of it. The missing share was theorized to hide as the warm-hot intergalactic medium (WHIM), gas at 10^5 to 10^7 K strung along cosmic web filaments, too cool for easy X-ray detection and too hot for absorption-line surveys. Localized fast radio bursts broke the impasse: each FRB dispersion measure integrates the free-electron column along its path, and the Macquart relation between dispersion and redshift, established first with seven events and now with large DSA-110-era samples, accounts for the full baryon budget, placing roughly 80 percent of baryons in the diffuse intergalactic phase (arXiv:2409.16952). eROSITA stacking now images WHIM X-ray emission in long filaments directly.
The census is closing, but the accounting exposes the model debt: ΛCDM spent twenty years unable to locate a third of its ordinary matter, and the resolution depends on feedback prescriptions, how violently galaxies eject gas, that remain the least constrained ingredient in simulations. The partition among IGM, circumgalactic halos, and clusters still varies among feedback models by tens of percent, and FRB constraints are now sharper than the simulations they test. A model whose visible sector was missing for decades, found only by a serendipitous new messenger, retains the burden of explaining why the baryons reside exactly where they do.
The standing is a success story with a live edge: thousands of localized FRBs this decade will turn the Macquart relation into a precision partition of cosmic baryons by environment, a measurement every theory of structure and feedback must now match in detail.