  We analyse the X-ray surface brightness profiles of 19 moderately distant and
luminous clusters of galaxies observed with the \EINOBS. Our aim is to
determine cluster gas masses out to radii between 1 and $3\Mpc$, and to confirm
the apparent conflict, if $\Omega_0=1$, between the current calculations of the
mean baryon fraction of the Universe expected from standard primordial
nucleosynthesis, and the fraction of the mass in clusters which is in gas. Our
analysis shows that baryon overdensities in clusters are much more widespread
than only the Coma cluster with which S.~White \& Frenk originally highlighted
this problem. The uncertainties involved in our analysis and some cosmological
implications from our results are briefly discussed. For a refined sample of 13
clusters we find that the baryon fraction for the gas within $1\Mpc$ lies
between 10 and 22 per cent.
