  More than three quarters of the baryonic content of the Universe resides in a
highly diffuse state that is difficult to observe, with only a small fraction
directly observed in galaxies and galaxy clusters. Censuses of the nearby
Universe have used absorption line spectroscopy to observe these invisible
baryons, but these measurements rely on large and uncertain corrections and are
insensitive to the majority of the volume, and likely mass. Specifically,
quasar spectroscopy is sensitive either to only the very trace amounts of
Hydrogen that exists in the atomic state, or highly ionized and enriched gas in
denser regions near galaxies. Sunyaev-Zel'dovich analyses provide evidence of
some of the gas in filamentary structures and studies of X-ray emission are
most sensitive to gas near galaxy clusters. Here we report the direct
measurement of the baryon content of the Universe using the dispersion of a
sample of localized fast radio bursts (FRBs), thus utilizing an effect that
measures the electron column density along each sight line and accounts for
every ionised baryon. We augment the sample of published arcsecond-localized
FRBs with a further four new localizations to host galaxies which have measured
redshifts of 0.291, 0.118, 0.378 and 0.522, completing a sample sufficiently
large to account for dispersion variations along the line of sight and in the
host galaxy environment to derive a cosmic baryon density of $\Omega_{b} =
0.051_{-0.025}^{+0.021} \, h_{70}^{-1}$ (95% confidence). This independent
measurement is consistent with Cosmic Microwave Background and Big Bang
Nucleosynthesis values.
