  Among the prominent candidates for dark matter are bosonic fields with small
scalar couplings to the Standard-Model particles. Several techniques are
employed to search for such couplings and the current best constraints are
derived from tests of gravity or atomic probes. In experiments employing atoms,
observables would arise from expected dark-matter-induced oscillations in the
fundamental constants of nature. These studies are primarily sensitive to
underlying particle masses below $10^{-14}$ eV. We present a method to search
for fast oscillations of fundamental constants using atomic spectroscopy in
cesium vapor. We demonstrate sensitivity to scalar interactions of dark matter
associated with a particle mass in the range $8\cdot10^{-11}$ to $4\cdot
10^{-7}$ eV. In this range our experiment yields constraints on such
interactions, which within the framework of an astronomical-size dark matter
structure, are comparable with, or better than, those provided by experiments
probing deviations from the law of gravity.
