  A long time ago (in 2014), in galaxies and galaxy clusters far, far away,
several groups have reported hints for a yet unidentified line in astrophysical
X-ray signals at an energy of 3.5\,keV. While it is not unlikely that this line
is simply a reflection of imperfectly modeled atomic transitions, it has
renewed the community's interest in models of keV-scale dark matter, whose
decay would lead to such a line. The alternative possibility of dark matter
annihilation into monochromatic photons is far less explored, a lapse that we
strive to amend in this paper. More precisely, we introduce a novel model of
fermionic dark matter $\chi$ with $\mathcal{O}(\text{keV})$ mass, annihilating
to a scalar state $\phi$ which in turn decays to photons, for instance via
loops of heavy vector-like quarks. The resulting photon spectrum is box-shaped,
but if $\chi$ and $\phi$ are nearly degenerate in mass, it can also resemble a
narrow line. We discuss dark matter production via two different mechanisms --
misalignment and freeze-in -- which both turn out to be viable in vast regions
of parameter space. We constrain the model using astrophysical X-ray data, and
we demonstrate that, thanks to the velocity-dependence of the annihilation
cross section, it has the potential to reconcile the various observations of
the 3.5\,keV line. We finally address the $\phi$-mediated force between dark
matter particles and its possible impact on structure formation.
