  We search for energetic electron recoil signals induced by boosted dark
matter (BDM) from the galactic center using the COSINE-100 array of NaI(Tl)
crystal detectors at the Yangyang Underground Laboratory. The signal would be
an excess of events with energies above 4 MeV over the well-understood
background. Because no excess of events are observed in a 97.7 kg$\cdot$years
exposure, we set limits on BDM interactions under a variety of hypotheses.
Notably, we explored the dark photon parameter space, leading to competitive
limits compared to direct dark photon search experiments, particularly for dark
photon masses below 4\,MeV and considering the invisible decay mode.
Furthermore, by comparing our results with a previous BDM search conducted by
the Super-Kamionkande experiment, we found that the COSINE-100 detector has
advantages in searching for low-mass dark matter. This analysis demonstrates
the potential of the COSINE-100 detector to search for MeV electron recoil
signals produced by the dark sector particle interactions.
