We reconstruct the nonlinear matter power spectrum $P(k)$ using a joint analysis of gravitational lensing of the cosmic microwave background (CMB) and lensing of galaxies. This reconstruction is motivated by the $S_8$ tension between early-universe CMB predictions and late-time observables. We use CMB lensing data from the Atacama Cosmology Telescope DR6 and cosmic shear data from the Dark Energy Survey (DES) Y3 release to perform a gravity-only (i.e., no baryonic feedback) fit to $P(k)$ in bins of wave number, within a model with a cosmological constant $\mathrm{\Lambda }$ and a cold dark matter component ($\mathrm{\Lambda }$CDM). We find that with DES cosmic shear data alone, $P(k)$ departs from the early-universe CMB prediction on all scales. The joint fit with CMB lensing is consistent on large scales $k<0.2{\mathrm{Mpc}}^{-1}$ but shows an $\sim 2\sigma$ deviation from scale independence when extending to $k=10h/\mathrm{Mpc}$. We compare our agnostic $P(k)$ reconstruction to baryonic feedback models and nonstandard dark matter models: Reasonable variations of both scenarios can recover the shape and amplitude of the suppression. We discuss the advances needed to disentangle these physical effects with a full mapping of $P(k,z)$.

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• Cosmic microwave background
• Large scale structure of the Universe