It has been known for decades that the observed number of baryons in the local Universe falls about 30–40 per cent short1,2 of the total number of baryons predicted3 by Big Bang nucleosynthesis, as inferred4,5 from density fluctuations of the cosmic microwave background and seen during the first 2–3 billion years of the Universe in the so-called ‘Lyman α forest’6,7 (a dense series of intervening H i Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm–hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas—hydrogen—is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios8,9. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far2,10. Here we report observations of two absorbers of highly ionized oxygen (O vii) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar’s intrinsic outflow or the host galaxy’s interstellar medium implausible. The O vii systems lie in regions characterized by large (four times larger than average11) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm–hot intergalactic medium. We conclude that the missing baryons have been found.
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This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. F.N. and M.E. acknowledge support from NASA grant NNX17AD76G. S.M. acknowledges NASA grant NNX16AF49G. S.B. acknowledges financial support through agreement ASI-INAF n.2017-14-H.0 and an INFN INDARK grant. Y.K. thanks INAOE for the support offered during a sabbatical visit in 2017 and acknowledges support from grant DGAPA-PAPIIT 106518 and from programme DGAPA-PASPA. R.C. acknowledges support from NSF grant AST-1515389.
Nature thanks R. Davé and T. Fang for their contribution to the peer review of this work.
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