Diffuse baryonic matter beyond 2020
  The hot, diffuse gas that fills the largest overdense structures in the
Universe -- clusters of galaxies and a web of giant filaments connecting them
-- provides us with tools to address a wide array of fundamental astrophysical
and cosmological questions via observations in the X-ray band. Clusters are
sensitive cosmological probes. To utilize their full potential for precision
cosmology in the following decades, we must precisely understand their physics
-- from their cool cores stirred by jets produced by the central supermassive
black hole (itself fed by inflow of intracluster gas), to their outskirts,
where the infall of intergalactic medium (IGM) drives shocks and accelerates
cosmic rays. Beyond the cluster confines lies the virtually unexplored warm
IGM, believed to contain most of the baryonic matter in the present-day
Universe. As a depository of all the matter ever ejected from galaxies, it
carries unique information on the history of energy and metal production in the
Universe. Currently planned major observatories, such as Astro-H and IXO, will
make deep inroads into these areas, but to see the most interesting parts of
the picture will require an almost science-fiction-grade facility with tens of
m^2 of effective area, subarcsecond angular resolution, a matching imaging
calorimeter and a super high-dispersion spectrograph, such as Generation-X.
