  Many experiments exploring weakly interacting massive particles (WIMPs) such
as direct, indirect and collider searches have been carried out until now.
However, a clear signal of a WIMP has not been found yet and it makes us to
suspect that WIMPs are questionable as a dark matter candidate. Taking into
account this situation, we propose two models in which dark matter relic
density is produced by decay of a metastable particle. In the first model, the
metastable particle is a feebly interacting massive particle, which is the
so-called FIMP produced by freeze-in mechanism in the early universe. In the
second model, the decaying particle is thermally produced the same as the usual
WIMP. However decay of the particle into dark matter is led by a higher
dimensional operator. As a phenomenologically interesting feature of nonthermal
dark matter discussed in this paper, a strong sharp gamma-ray emission as an
indirect detection signal occurs due to internal bremsstrahlung, although some
parameter space has already been ruled out by this process. Moreover combining
other experimental and theoretical constraints such as dark matter relic
density, big bang nucleosynthesis, collider, gamma-rays and perturbativity of
couplings, we discuss the two nonthermal DM models.
