  Recently two groups independently observed unidentified X-ray line signal at
the energy 3.55 keV from the galaxy clusters and Andromeda galaxy. We show that
this anomalous signal can be explained in annihilating dark matter model, for
example, fermionic dark matter model in hidden sector with global $U(1)_X$
symmetry proposed by Weinberg. There are two scenarios for the production of
the annihilating dark matters. In the first scenario the dark matters with mass
3.55 keV decouple from the interaction with Goldstone bosons and go out of
thermal equilibrium at high temperature ($>$ 1 TeV) when they are still
relativistic, their number density per comoving volume being essentially fixed
to be the current value. The correct relic abundance of this warm dark matter
is obtained by assuming that about ${\cal O}(10^3)$ relativistic degrees of
freedom were present at the decoupling temperature or alternatively large
entropy production occurred at high temperature. In the other scenario, the
dark matters were absent at high temperature, and as the universe cools down,
the SM particles annihilate or decay to produce the dark matters non-thermally
as in `freeze-in' scenario. It turns out that the DM production from Higgs
decay is the dominant one. In the model we considered, only the first scenario
can explain both X-ray signal and relic abundance. The X-ray signal arises
through $p$-wave annihilation of dark matter pair into two photons through the
scalar resonance without violating the constraints from big bang
nucleosynthesis, cosmic microwave background, and astrophysical objects such as
red giants or white dwarfs. We also discuss the possibility that the signal may
result from a decaying dark matter in a simple extension of Weinberg model.
