  Controlling matter-light interactions with cavities is of fundamental
importance in modern science and technology. It is exemplified in the
strong-coupling regime, where matter-light hybrid modes form, with properties
controllable via the photon component on the optical-wavelength scale. In
contrast, matter excitations on the nanometer scale are harder to access. In
two-dimensional van der Waals heterostructures, a tunable moir\'e lattice
potential for electronic excitations may form, enabling correlated electron
gases in lattice potentials. Excitons confined in moir\'e lattices have also
been reported, but cooperative effects have been elusive and interactions with
light have remained perturbative. Here, integrating MoSe$_{2}$-WS$_{2}$
heterobilayers in a microcavity, we establish cooperative coupling between
moir\'e-lattice excitons and microcavity photons up to liquid-nitrogen
temperature, thereby integrating into one platform versatile control over both
matter and light. The density dependence of the moir\'e polaritons reveals
strong nonlinearity due to exciton blockade, suppressed exciton energy shift,
and suppressed excitation-induced dephasing, all of which are consistent with
the quantum-confined nature of the moir\'e excitons. Such a moir\'e polariton
system combines strong nonlinearity and microscopic-scale tuning of matter
excitations with the power of cavity engineering and long range coherence of
light, providing a new platform for collective phenomena from tunable arrays of
quantum emitters.
