  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 Sunyaev-Zel'dovich 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 >0.1eV. 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 >5 after a day of integration
with the future Constellation-X telescope for any of the several hundred
brightest quasars across the sky.
