  Axino and gravitino are promising candidates to solve the dark matter (DM)
problem in the framework of supersymmetry. In this work, we assume that the
axino is the lightest supersymmetric particle (LSP), and therefore contributes
to DM. In the case of R-parity violating models, the axino can decay into a
neutrino-photon pair with a lifetime much longer than the age of the Universe,
yielding a potentially detectable signal. Interestingly, a gravitino
next-to-LSP (NLSP) can live enough as to contribute to the relic density. We
study both scenarios, only axino LSP as DM, and axino LSP with gravitino NLSP
as DM. We carry out the analysis in the context of the $\mu\nu$SSM, which
solves the $\mu$ problem and reproduces neutrino data, only adding couplings
involving right-handed neutrinos. In particular, we perform a complete analysis
of the relevant parameter space of the model considering constraints from
neutrino physics, cosmological observations, and $\gamma$-ray detection. We
find that the axino or the gravitino can produce a signal detectable by future
MeV-GeV $\gamma$-ray telescopes. In addition, in a parameter region where we
get a well-tempered mixture of both particles, a double-line signal arises as a
smoking gun.
