  Stars and gas in galaxies, hot intracluster medium, and intergalactic
photo-ionized gas make up at most half of the baryons that are expected to be
present in the universe. The majority of baryons are still missing and are
expected to be hidden in a web of warm-hot intergalactic medium. This matter
was shock-heated during the collapse of density perturbations that led to the
formation of the relaxed structures that we see today. Finding the missing
baryons and thereby producing a complete inventory of possibly the only
detectable component of the energy-mass budget of the universe is crucial to
validate or invalidate our standard cosmological model.
