Properties that distinguish alternative models from CDM. Left: Input matter power spectrum P(k) in CDM and WDM models at the initial time of the simulations (z = 127). Warmer models show a cut-off at increasingly larger scales, corresponding to smaller k values. Right: Self-interaction cross-section σ/m_χ as a function of velocity. We consider a model with a constant cross-section (blue line) and one with a steep velocity dependence (green line) from Correa (2021).