About half of the 5% of regular (ie baryonic) Matter was missing from usual astronomical observations. Some hypothesized it took the form of warm-hot intergalactic gas. This could in principle be detected by looking at the dispersion (ie slightly varying transit speeds of waves) through this gas of fast radio burst from distant galaxies, and we just found like five of these FRBs that showed dispersions that matched the predicted amount of baryonic warm-hot gas. Problem solved.