  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, H1821+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 OVII
absorption line that exhibits a Gaussian line profile and is statistically
significant at the $3.3 \sigma$ 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
OVII line is $(4.1\pm1.3) \ \mathrm{m\AA}$, which corresponds to an OVII column
density of $(1.4\pm0.4)\times10^{15} \ \mathrm{cm^{-2}}$. We constrain the
absorbing gas to have a density of $n_{\rm H} = (1-2)\times10^{-6} \
\rm{cm^{-3}}$ for a single WHIM filament. We derive $\Omega_{\rm b} \rm(O\,VII)
= (0.0023 \pm 0.0007) \, \left[ f_{O\,VII} \, {Z/Z_{\odot}} \right]^{-1}$ for
the cosmological mass density of OVII, assuming that all 17 systems contribute
equally.
