Strong gravitational lensing is a powerful tool to unveil the inner structure of galaxy clusters. Mapping the distribution of matter in these systems’ cores is essential to investigate structure formation, test the nature of dark matter, and study the interplay between dark matter and baryons. I will discuss some recent work to build accurate models of the mass distributions in few galaxy clusters combining observations with Hubble and MUSE at the VLT. Then, I will discuss how the mass reconstructions compare to numerical simulations in the LCDM cosmological model framework. Such comparison recently revealed a significant discrepancy between the expected and the observed number of strong lensing events associated with cluster galaxies. Such difference indicates that galaxy-scale dark matter substructures in galaxy clusters are more compact than theoretically expected. Systematic issues with simulations or incorrect assumptions about the properties of dark matter could explain these results.