In $\Lambda$CDM, dark matter halos follow a well-defined concentration-mass ($c-M$) relation, where lower-mass halos are denser (more concentrated) because they form earlier when the universe was denser. However, observations of galaxies and clusters reveal a much larger scatter and systematic deviations from this prediction. Many dwarf galaxies exhibit "cores" (low concentration) where cusps are expected, while some massive clusters are surprisingly over-concentrated. This "diversity problem" suggests that halo structure is not governed solely by gravity and mass accretion history, but depends on unknown factors that break the universality of the NFW profile (Bullock & Boylan-Kolchin 2017; Kaplinghat et al. 2016).