Standard $\Lambda$CDM simulations predict a specific abundance and radial distribution of subhalos within galaxy clusters. However, strong lensing observations consistently reveal a significant excess of small-scale substructures (subhalos) in the inner regions of clusters compared to these predictions. Furthermore, the observed subhalos are often more compact and concentrated than simulated ones, surviving tidal destruction where they should have been disrupted. This "substructure crisis" suggests that real dark matter structures are more resilient or numerous than collisionless CDM allows (Meneghetti et al. 2020; Natarajan et al. 2017).