  We present an estimate of the X-ray background (XRB) spectrum from the
warm-hot intergalactic medium (IGM) associated with groups and clusters of
galaxies, using purely the observationally determined X-ray luminosity function
(XLF) and X-ray luminosity (Lx) - temperature (T) relations for groups and
clusters. As compared with previous semi-analytic models based on the
Press-Schechter mass function, our approach provides a much simpler and more
realistic way to evaluate the XRB from groups and clusters in the sense that we
make no assumption about the dynamical and heating properties of the IGM, and
the intrinsic dispersion in the Lx-T relations due to different physical
mechanisms among different groups and clusters can be also included. It shows
that the resulting 0.1-10 keV XRB spectrum by summing up the X-ray emission
from all groups and clusters is consistent with current upper limits placed on
the contribution from diffuse gas to the XRB. This may have profound
implications for our understanding of the missing baryons in the universe.
