The current census of observed baryons in the local universe is still missing a significant fraction of them according to standard big bang nucleosynthesis. Numerical simulations predict that most of the missing baryons are in a hot intergalactic medium, which is difficult to observe through its X-ray emission or the Sunyaev-Zeldovich effect. We show that the next generation of X-ray satellites will be able to detect this gas through the X-ray absorption lines imprinted by its highly ionized metals on the spectrum of a background quasar. For the metallicity typically found in intracluster gas, up to 70% of the baryons produce O VIII absorption lines with an equivalent width >~0.1 eV. The spectrum of any high-redshift quasar is expected to show several such lines per unit redshift due to intervening gaseous halos of galaxy groups. These lines will be detectable at a signal-to-noise ratio of >~5 after a day of integration with the future Constellation-X telescope for any of the several tens of the brightest quasars across the sky.