  The cluster baryon fraction is estimated from the CMB-scattering leptonic
component of the intracluster medium (ICM); however, the observed cluster
baryon fraction is less than the cosmic one. Understanding the origin of this
discrepancy is necessary for correctly describing the structure of the ICM. We
estimate the baryonic mass in the outskirts of galaxy clusters which is
difficult to observe because of low electron temperature and density in these
regions. The time scale for the electrons and protons to reach equipartition in
the outskirts is longer than the cluster age. Since thermal equilibrium is not
achieved, a significant fraction of the ICM baryons may be hidden in shells
around galaxy clusters. We derive the necessary condition on the cluster mass
for the concealment of missing baryons in an outer baryon shell and show that
this condition is fulfilled because cluster masses are comparable to the
estimated characteristic mass $M=e^4/(m^3_{p} G^2)=1.3x10^{15}$ solar masses.
The existence of extreme-ultraviolet emission haloes around galaxy clusters is
predicted.
