We analyse the X-ray surface brightness profiles of 19 moderately distant and luminous clusters of galaxies observed with the Einstein Observatory .
Our aim is to determine cluster gas masses out to radii between 1 and 3 { \thinspace 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 { \thinspace Mpc } lies between 10 and 22 per cent .