  The role of cosmic rays (CR) in the formation and evolution of clusters of
galaxies has been much debated. It may well be related to other fundamental
questions, such as the mechanism which heats and virializes the intracluster
medium (ICM), and the frequency at which the ICM is shocked. There is now
compelling evidence both from the cluster soft excess (CSE) and the `hard-tail'
emissions at energies above 10 keV, that many clusters are luminous sources of
inverse-Compton (IC) emission. This is the first direct measurement of cluster
CR: the technique is free from our uncertainties in the ICM magnetic field, and
is not limited to the small subset of clusters which exhibit radio halos. The
CSE emitting electrons fall within a crucial decade of energy where they have
the least spectral evolution, and where most of the CR pressure resides.
However their survival times do not date them back to the relic CR population.
By using the CSE data of the Coma cluster, we demonstrate that the CR are
energetically as important as the thermal ICM: the two components are in
pressure equiparition. Thus, contrary to previous expectations, CR are a
dominant component of the ICM, and their origin and effects should be explored.
The best-fit CR spectral index is in agreement with the Galactic value.
