  The properties of hydrogen under extreme conditions are important for many
applications, including inertial confinement fusion and astrophysical models. A
key quantity is given by the electronic density response to an external
perturbation, which is probed in X-ray Thomson scattering (XRTS) experiments --
the state of the art diagnostics from which system parameters like the free
electron density $n_e$, the electronic temperature $T_e$, and the charge state
$Z$ can be inferred. In this work, we present highly accurate path integral
Monte Carlo (PIMC) results for the electronic density response of hydrogen. We
obtain the exchange-correlation (XC) kernel $K_{xc}$, which is of central
relevance for many applications, such as time-dependent density functional
theory (TD-DFT). This gives us a first unbiased look into the electronic
density response of hydrogen in the warm-dense matter regime, thereby opening
up a gamut of avenues for future research.
