  The backbone of the large-scale structure of the Universe is determined by
processes on a cosmological scale and by the gravitational interaction of the
dominant dark matter. However, the mobile baryon population shapes the
appearance of these structures. Theory predicts that most of the baryons reside
in vast unvirialized filamentary structures that connect galaxy groups and
clusters, but the observational evidence is currently lacking. Because the
majority of the baryons are supposed to exist in a large-scale, hot and dilute
gaseous phase, X-rays provide the ideal tool to progress our understanding.
Observations with the Athena+ X-ray Integral Field Unit will reveal the
location, chemical composition, physical state and dynamics of the active
population of baryons.
