  A large fraction of the baryons today are predicted to be in hot, filamentary
gas, which has yet to be detected. In this paper, we use numerical simulations
of dark matter and gas to determine if these filaments and groups of galaxy
will be observable be XMM. The simulated maps include free-free and line
emission, galactic absorption, the XMM response function, photon noise, the
extragalactic source distribution, and vignetting. A number of cosmological
models are examined as well as a range of very simple prescriptions to account
for the effect of supernovae feedback (preheating). We show that XMM has a good
chance of observing emission from strong filaments at z~0.1. This becomes much
more difficult by z~0.5. The primary difficulties lie in detecting such a
large, diffuse object and in selecting an appropriate field. We also describe
the range of group sizes that XMM should be sensitive to (both for detection
and spectral analysis), although this is more dependent on the unknown nature
of the feedback. Such observations will greatly improve our understanding of
feedback and should also provide stronger cosmological constraints.
