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 O vii ( 561 , 568 , 574 , 665eV ) and O viii ( 653eV ) .
For this purpose , we create mock spectra of the emission lines of WHIM using a light-cone output of cosmological hydrodynamic simulations .
While the predicted fluxes are generally below the current detection limit , an unambiguous detection will be feasible with a dedicated X-ray satellite mission that we also discuss in detail .
Our proposed mission is especially 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 .