To reconcile the standard cosmological model with the X-ray measured number of galaxy clusters, a team of French scientists has shown that the cluster masses should be increased by 70% compared to current estimates. These results follow the conclusions obtained by the Planck mission in 2013 from observations of galaxy clusters in the microwave domain. This huge difference is intriguing: either our understanding of the physics of galaxy clusters needs to be revised, or the standard cosmological model is incomplete.