  (Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict
the extragalactic X-ray background (XRB), focussing on emission from the
intergalactic medium (IGM). We also include X-rays from point sources
associated with galaxies in the simulation, and make maps of the angular
distribution of the emission. We find that filaments in the maps are not
evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean
intensity of radiation from intergalactic and cluster gas is 2.3*10^-12
ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at
the ~1 sigma level with estimates of the unresolved soft background from ROSAT
and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with
the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift
(median z~0.45) so its clustering makes an important contribution to that of
the total XRB. The angular correlations on 0.1-10 arcmin scales are
significant, with an amplitude roughly consistent with an extrapolation of
recent ROSAT results to small scales. A cross-correlation of the XRB against
nearby galaxies taken from a simulated redshift survey also yields a strong
signal from the IGM. Although some recent papers have argued that the expected
soft band intensity from gas in galaxy, group, and cluster halos would exceed
XRB limits unless much of the gas is expelled by supernova feedback, we obtain
reasonable compatibility with current observations in a simulation that
incorporates cooling, star formation, and only modest feedback. A prediction of
our model is that the unresolved portion of the soft XRB will remain mostly
unresolved.
