  Spatial variations in the cosmic electron density after reionization generate
cosmic microwave background anisotropies via Thomson scattering, a process
known as the ``patchy screening" effect. In this paper, we propose a new
estimator for the patchy screening effect that is designed to mitigate biases
from the dominant foreground signals. We use it to measure the
cross-correlation between \textit{unWISE} galaxies and patchy screening, the
latter measured by the Atacama Cosmology Telescope and \textit{Planck}
satellite. We report the first detection of the patchy screening effect, with
the statistical significance of the cross-correlation exceeding $7\sigma$. This
measurement directly probes the distribution of electrons around these galaxies
and provides strong evidence that gas is more extended than the underlying dark
matter. By comparing our measurements to electron profiles extracted from
simulations, we demonstrate the power of these observations to constrain galaxy
evolution models. Requiring only the 2D positions of objects and no individual
redshifts or velocity estimates, this approach is complementary to existing gas
probes, such as those based on the kinetic Sunyaev-Zeldovich effect.
