  The X-ray Background (XRB) probably originates from the integrated X-ray
emission of active galactic nuclei (AGN). Modelling of its flat spectrum
implies considerable absorption in most AGN. Compton down-scattering means that
sources in which the absorption is Compton thick are unlikely to be major
contributors to the background intensity so the observed spectral intensity at
about 30 keV is little affected by photoelectric absorption. Assuming that the
intrinsic photon index of AGN is 2, we then use the 30 keV intensity of the XRB
to infer the absorption-corrected energy density of the background. Soltan's
argument then enables us to convert this to a mean local density in black
holes, assuming an accretion efficiency of 0.1 and a mean AGN redshift of 2.
The result is within a factor of two of that estimated by Haehnelt et al from
the optically-determined black hole masses of Magorrian et al. We conclude that
there is no strong need for any radiatively inefficient mode of accretion for
building the masses of black holes. Furthermore we show that the absorption
model for the XRB implies that about 85 per cent of accretion power in the
Universe is absorbed. This power probably emerges in the infrared bands where
it can be several tens per cent of the recently inferred backgrounds there. The
total power emitted by accretion is then about one fifth that of stars.
