The cosmic X-ray background: abundance and evolution of hidden black
  holes
  The growth of supermassive black holes across cosmic time leaves a radiative
imprint recorded in the X-ray background (XRB). The XRB spectral shape suggests
that a large population of distant, hidden nuclei must exist, which are now
being revealed at higher and higher redshifts by the deepest surveys performed
by Chandra and XMM.
  Our current understanding of the XRB emission in terms of AGN population
synthesis models is here reviewed, and the evolutionary path of nuclear
accretion and obscuration, as emerging from the major X-ray surveys, is
investigated. The role of galaxy merging versus secular processes in triggering
nuclear activity is also discussed in the framework of recent galaxy/black hole
co-evolutionary scenarios. Finally, the limits of current instrumentation in
the detection of the most obscured and distant black holes are discussed and
some possible directions to overcome these limits are presented.
