  We discuss the use of optical cavities as tools to search for dark matter
(DM) composed of virialized ultra-light fields (VULFs). Such fields could lead
to oscillating fundamental constants, resulting in oscillations of the length
of rigid bodies. We propose searching for these effects via differential strain
measurement of rigid and suspended-mirror cavities. We estimate that more than
two orders of magnitude of unexplored phase space for VULF DM couplings can be
probed at VULF Compton frequencies in the audible range of 0.1-10 kHz.
