  The characteristic excitation of a metal is its plasmon, which is a quantized
collective oscillation of its electron density. In 1956, David Pines predicted
that a distinct type of plasmon, dubbed a "demon," could exist in
three-dimensional metals containing more than one species of charge carrier.
Consisting of out-of-phase movement of electrons in different bands, demons are
acoustic, electrically neutral, and do not couple to light, so have never been
detected in an equilibrium, three-dimensional metal. Nevertheless, demons are
believed to be critical for diverse phenomena including phase transitions in
mixed-valence semimetals, optical properties of metal nanoparticles,
"soundarons" in Weyl semimetals, and high temperature superconductivity in, for
example, metal hydrides. Here, we present evidence for a demon in Sr$_2$RuO$_4$
from momentum-resolved electron energy-loss spectroscopy (M-EELS). Formed of
electrons in the $\beta$ and $\gamma$ bands, the demon is gapless with a room
temperature velocity $v=1.065 \pm 0.12 \times 10^5$ m/s and critical momentum
$q_c=0.08$ reciprocal lattice units. Its spectral weight violates low-energy
partial sum rules, affirming its neutral character. Our study confirms a
66-year old prediction and suggests that demons may be a pervasive feature of
multiband metals.
