  We propose a new search channel for boosted dark matter (BDM) signals coming
from the present universe, which are distinct from simple neutrino signals
including those coming from the decay or pair-annihilation of dark matter. The
signal process is initiated by the scattering of high-energetic BDM off either
an electron or a nucleon. If the dark matter is dark-sector U(1)-charged, the
scattered BDM may radiate a dark gauge boson (called "dark-strahlung") which
subsequently decays to a Standard Model fermion pair. We point out that the
existence of this channel may allow for the interpretation that the associated
signal stems from BDM, not from the dark-matter-origin neutrinos. Although the
dark-strahlung process is generally subleading compared to the lowest-order
simple elastic scattering of BDM, we find that the BDM with a significant boost
factor may induce an O(10-20%) event rate in the parameter regions unreachable
by typical beam-produced dark-matter. We further find that the dark-strahlung
channel may even outperform the leading-order channel in the search for BDM,
especially when the latter is plagued by substantial background contamination.
We argue that cosmogenic BDM searches readily fall in such a case, hence taking
full advantage of dark-strahlung. As a practical application, experimental
sensitivities expected in the leading-order and dark-strahlung channels are
contrasted in dark gauge boson parameter space, under the environment of DUNE
far-detectors, revealing usefulness of dark-strahlung.
