  Based on 19 high-resolution N-body/gas-dynamical galaxy formation simulations
in the LCDM cosmology it is shown, that for a galaxy like the Milky Way, in
addition to the baryonic mass of the galaxy itself, about 70% extra baryonic
mass should reside around the galaxy (inside of the virial radius), chiefly in
the form of hot gas. Averaging over the entire field galaxy population, this
``external'' component amounts to 64-85% of the baryonic mass of the population
itself. These results are supported by the recent detection of very extended,
soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC
5746.
  Some of the hot gas may, by thermal instability, have condensed into mainly
pressure supported, warm clouds, similar to the Galactic High Velocity Clouds
(HVCs). Based on an ultra-high resolution cosmological test simulation of a
Milky Way like galaxy (with a gas particle mass and gravity softening length of
only 7600 h^-1 Msun and 83 h^-1 pc,respectively), it is argued, that the hot
gas phase dominates over the warm gas phase, in the halo. Finally, an origin of
HVCs as ``leftovers'' from filamentary, ``cold'' accretion events, mainly
occurring early in the history of galaxies, is proposed.
