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More than three quarters of the baryonic content of the Universe resides in ahighly diffuse state that is difficult to observe, with only a small fractiondirectly observed in galaxies and galaxy clusters. Censuses of the nearbyUniverse have used absorption line spectroscopy to observe these invisiblebaryons, but these measurements rely on large and uncertain corrections and areinsensitive to the majority of the volume, and likely mass. Specifically,quasar spectroscopy is sensitive either to only the very trace amounts ofHydrogen that exists in the atomic state, or highly ionized and enriched gas indenser regions near galaxies. Sunyaev-Zel'dovich analyses provide evidence ofsome of the gas in filamentary structures and studies of X-ray emission aremost sensitive to gas near galaxy clusters. Here we report the directmeasurement of the baryon content of the Universe using the dispersion of asample of localized fast radio bursts (FRBs), thus utilizing an effect thatmeasures the electron column density along each sight line and accounts forevery ionised baryon. We augment the sample of published arcsecond-localizedFRBs with a further four new localizations to host galaxies which have measuredredshifts of 0.291, 0.118, 0.378 and 0.522, completing a sample sufficientlylarge to account for dispersion variations along the line of sight and in thehost galaxy environment to derive a cosmic baryon density of $\Omega_{b} =0.051_{-0.025}^{+0.021} \, h_{70}^{-1}$ (95% confidence). This independentmeasurement is consistent with Cosmic Microwave Background and Big BangNucleosynthesis values.

A census of baryons in the Universe from localized fast radio bursts