  Approximately 30-40% of all baryons in the present day universe reside in a
warm-hot intergalactic medium (WHIM), with temperatures between 10^5<T<10^7 K.
This is a generic prediction from six hydrodynamic simulations of currently
favored structure formation models having a wide variety of numerical methods,
input physics, volumes, and spatial resolutions. Most of these warm-hot baryons
reside in diffuse large-scale structures with a median overdensity around
10-30, not in virialized objects such as galaxy groups or galactic halos. The
evolution of the WHIM is primarily driven by shock heating from gravitational
perturbations breaking on mildly nonlinear, non-equilibrium structures such as
filaments. Supernova feedback energy and radiative cooling play lesser roles in
its evolution. WHIM gas is consistent with observations of the 0.25 keV X-ray
background without being significantly heated by non-gravitational processes
because the emitting gas is very diffuse. Our results confirm and extend
previous work by Cen & Ostriker and Dave' et al.
