We use a large, statistically very well characterized X-ray flux-limited sample of galaxy clusters to study the large-scale structure of the Universe out to redshifts of ~ 0.4. From the cluster mass function we obtain tight constraints on the matter density and amplitude parameter of the density fluctuation power spectrum. We find some tension in the resulting amplitude parameter with the prediction from the PLANCK results in the frame of the standard Lambda-CDM cosmological model. The tension implies a less pronounced fluctuation amplitude of nearby large-scale structure as compared to the predictions based on Planck and a pure LCDM model. The results can be reconciled, however, by for example introducing massive neutrinos. We also use the cluster sample to study the matter distribution in the local Universe. One of the findings of this study is a locally underdense region in the Southern Galactic Cap, with interesting consequences for local measurements of cosmological parameters, like the Hubble constant.