  A new cosmological scenario is proposed in which a light scalaron of $f (R)$
gravity plays the role of dark matter. In this scenario, the scalaron initially
resides at the minimum of its effective potential while the electroweak
symmetry is unbroken. At the beginning of the electroweak crossover, the
evolving expectation value of the Higgs field triggers the evolution of the
scalaron due to interaction between these fields. After the electroweak
crossover, the oscillating scalaron can represent cold dark matter. Its current
energy density depends on a single free parameter, the scalaron mass $m$, and
the value $m \simeq 4 \times 10^{-3}\, \text{eV}$ is required to explain the
observed dark-matter abundance. Larger mass values would be required in
scenarios where the scalaron is excited before the electroweak crossover.
