  Collective strong coupling between a disordered ensemble of $N$ localized
molecular vibrations and a resonant optical cavity mode gives rise to 2
polariton and $N-1\gg2$ dark modes. Thus, experimental changes in
thermally-activated reaction kinetics due to polariton formation appear
entropically unlikely and remain a puzzle. Here we show that the overlooked
dark modes, while parked at the same energy as bare molecular vibrations, are
robustly delocalized across $\sim$2-3 molecules, yielding enhanced channels of
vibrational cooling, concomitantly catalyzing a chemical reaction. As an
illustration, we theoretically show a $\approx$50\% increase in an electron
transfer rate due to enhanced product stabilization. The reported effects can
arise when the homogeneous linewidths of the dark modes are smaller than their
energy spacings.
