  We demonstrate the long range strong coupling of magnetostatic modes in
spatially separated ferromagnets mediated by a microwave frequency cavity. Two
spheres of yttrium iron garnet are embedded in the cavity and their
magnetostatic modes probed using a dispersive measurement technique. We find
they are strongly coupled to each other even when detuned from the cavity
modes. We investigate the dependence of the magnet-magnet coupling on the
cavity detuning $\Delta$, and find a $1/\Delta$ dependence also characteristic
of cavity-coupled superconducting qubits. Dark states of the coupled
magnetostatic modes of the system are observed, and ascribed to mismatches
between the symmetries of the modes and the drive field.
