  The abundance of local clusters is a traditional way to derive the amplitude
of matter fluctuations. In the present work, by assuming that the observed
baryon content of clusters is representative of the universe, we show that the
mass temperature relation (M-T) can be specified for any cosmological model.
This approach allows one to remove most of the uncertainty coming from M-T
relation, and to provide an estimation of sigma\_8 whose uncertainty is
essentially statistical. The values we obtain are fortuitously almost
independent of the matter density of the Universe (sigma\_8 ~ 0.6-0.63) with an
accuracy better than 5%. Quite remarkably, the amplitude of matter fluctuations
can be also tightly constrained to similar accuracy from existing CMB
measurements alone. However, the amplitude inferred in this way in a
concordance model (Lambda-CDM) is significantly larger than the value derived
from the above method based on X-ray clusters. Such a discrepancy would almost
disappear if the actual optical thickness of the Universe was 0 but could also
be alleviated from more exotic solutions: the existence of a new dark component
in the Universe as massive neutrinos. However, recent other indications of
sigma\_8 favor a high normalization. In this case, the assumption that the
baryonic content observed in clusters actually reflects the primordial value
has to be relaxed : either there exists a large baryonic dark component in the
Universe or baryons in clusters have undergone a large depletion during the
formation of these structures. We concluded that the baryon fraction in
clusters is not representative and therefore that an essential piece of the
physics of baryons in clusters is missing in standard structure formation
scenario.
