  Most of cosmic baryons predicted by the big-bang nucleosynthesis has evaded
the direct detection. Recent numerical simulations indicate that approximately
30 to 50 percent of the total baryons in the present universe is supposed to
take a form of warm/hot intergalactic medium (WHIM) whose X-ray continuum
emission is very weak. To identify those missing baryons, we consider in detail
the detectability of WHIM directly through emission lines of OVII (561, 568,
574, 665eV) and OVIII (653eV). For this purpose, we create mock spectra of the
emission lines of WHIM using a light-cone output of the cosmological
hydrodynamic simulations. Since the predicted fluxes are generally below the
current detection limit, the unambiguous detection requires a dedicated X-ray
satellite mission that we also discuss in detail. We find that our proposed
mission is sensitive to the WHIM with gas temperature $T=10^{6-7}$K and
overdensity $\delta=10-100$ up to a redshift of 0.3 without being significantly
contaminated by the cosmic X-ray background and the Galactic emissions. Thus
such a mission provides a unique and important tool to identify a large
fraction of otherwise elusive baryons in the universe.
