Simulations of the structural properties of massive DM halos in the CDM and SIDM frameworks. Left-hand panel: Einasto shape parameter α_Einasto for individual simulated halos with a varying SIDM cross section (0.1 cm² g−1, cyan; 0.3, purple; and 1.0, red; Robertson et al. 2021). For comparison, we also show the effect of varying baryonic physics (reference model, black; models with lower (yellow) and higher (green) feedback efficiency; McCarthy et al. 2017). Right-hand panel: median and 1σ percentiles of α_Einasto values for the various simulations runs.