  The origin of the 3.55 keV excess reported in various galaxies and galaxy
clusters remains unknown. Non-observations of the line in dwarf spheroidal
galaxies (dSphs) create tension with galaxy cluster observations when
considering a direct Dark Matter (DM) $\rightarrow \gamma$ decay. This
discrepancy could be explained by a DM $\rightarrow a \rightarrow \gamma$
decay, where the DM particles decay to axion-like particles (ALPs), which then
convert to photons in a magnetic field. The 3.55 keV line from a decay to ALPs
therefore has very different morphology from a direct decay to photons, as it
would only appear in environments with substantial magnetic fields between the
source and us. To test this hypothesis we consider targets where the line
strength would be enhanced compared to the DM $\rightarrow \gamma$ model. The
giant lobes of radio galaxies could represent good environments, having
magnetic fields with strength $\mathcal{O}(\mu\rm{G})$ that extend for hundreds
of kiloparsecs, and minimal X-ray background. In the case of Centaurus A, a
large population of dSphs are associated with the galaxy, some of which are
potentially in or behind its lobes. In this paper we estimate the flux of a
3.55 keV DM $\rightarrow a \rightarrow \gamma$ line from these sources, and
compare it to the flux from the DM halo of Centaurus~A. We comment on the
potential to detect such a signal with current satellites (such as XMM-Newton
and Chandra) and future satellites (such as Athena).
