Baryon fraction in clusters , combined with constraints from primordial nucleosynthesis is currently used to provide a robust upper limit on the cosmological density parameter \Omega _ { 0 } .
Current analyses lead to gas fractions at virial radius which are typically of the order of 0.20 h ^ { -3 / 2 } _ { 50 } , favoring a low density universe .
In this work , we examine critically this issue through the analysis of the baryon distribution in clusters .
We find that the currently derived gas fraction profile , increases regularly from the inner part to the outer part , up to the virial radius , and beyond .
Such a shape contrasts with what is expected from numerical hydro-dynamical simulations , in which the gas fraction is more or less constant in the outer region , reaching a plateau when the contrast density falls off below 10 ^ { 4 } .
We argue that such a difference is hardly explained by reheating effects , while taking into account various factors entering into the determination of clusters gas fraction may erase such a difference .
Indeed , using recent estimates on gas content in the outer part of clusters ( Vikhlinin et al. , 1999 ) and applying the correction factor due to the effect of gas clumping , we find that the gas fraction shape over the range 200 < \delta < 10 ^ { 5 } is roughly consistent with hydro-dynamical simulations for an universal gas fraction in the range 8 - 11 % ( for h _ { 50 } = 1. ) , the mass estimators being calibrated from numerical simulations .
In contrast , values of the order of 20 % do not give acceptable fit to the data on any scale .
We conclude , that high values of \Omega _ { 0 } can not be ruled out on the basis of the baryon fraction argument .