X-Ray Absorption by the Hot Intergalactic Medium
  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. 
Rosalba Perna and Abraham Loeb (Harvard University)