Based on constraints from Big Bang nucleosynthesis and the cosmic microwave background, the baryon content of the high-redshift Universe can be precisely determined. However, at low redshift, about one-third of the baryons remain unaccounted for, which poses the long-standing missing baryon problem. The missing baryons are believed to reside in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). In this work, we employ a novel stacking approach to explore the hot phases of the WHIM. Specifically, we utilize the 470 ks Chandra LETG data of the luminous quasar, H 1821 643, along with previous measurements of UV absorption line systems and spectroscopic redshift measurements of galaxies toward the quasar’s sightline. We repeatedly blueshift and stack the X-ray spectrum of the quasar corresponding to the redshifts of the 17 absorption line systems. Thus, we obtain a stacked spectrum with 8.0 Ms total exposure, which allows us to probe X-ray absorption lines with unparalleled sensitivity. Based on the stacked data, we detect an O VII absorption line that exhibits a Gaussian line profile and is statistically significant at the 3.3σ level. Since the redshifts of the UV absorption line systems were known a priori, this is the first definitive detection of an X-ray absorption line originating from the WHIM. The equivalent width of the O VII line is (4.1 ± 1.3) mÅ, which corresponds to an O VII column density of (1.4 /- 0.4)× {10}15 {cm}}-2. We constrain the absorbing gas to have a density of {n}{{H}}=(1-2)× {10}-6 {cm}}-3 for a single WHIM filament. We derive {{{Ω }}}{{b}}({{O}} {{VII}})=(0.0023 /- 0.0007){≤ft[{f}{{O}{{VII}}}Z/{Z}ȯ \right]}-1 for the cosmological mass density of O VII, assuming that all 17 systems contribute equally.
