  We propose an experiment to search for ultralight scalar dark matter (DM)
with dilatonic interactions. Such couplings can arise for the dilaton as well
as for moduli and axion-like particles in the presence of CP violation.
Ultralight dilaton DM acts as a background field that can cause tiny but
coherent oscillations in Standard Model parameters such as the fine structure
constant and the proton-electron mass ratio. These minute variations can be
detected through precise frequency comparisons of atomic clocks. Our experiment
extends current searches for drifts in fundamental constants to the
well-motivated high-frequency regime. Our proposed setups can probe scalars
lighter than 10^-15 eV with discovery potential of dilatonic couplings as weak
as 10^-11 times the strength of gravity, improving current equivalence
principle bounds by up to 8 orders of magnitude. We point out potential 10^4
sensitivity enhancements with future optical and nuclear clocks, as well as
possible signatures in gravitational wave detectors. Finally, we discuss
cosmological constraints and astrophysical hints of ultralight scalar DM, and
show they are complimentary to and compatible with the parameter range
accessible to our proposed laboratory experiments.
