  The current accelerated expansion of our universe could be due to an unknown
energy component with negative pressure (dark energy) or a modification to
general relativity (modified gravity). On the other hand, recently warm dark
matter (WDM) remarkably rose as an alternative of cold dark matter (CDM).
Obviously, it is of interest to distinguish these different types of models. In
fact, many attempts have been made in the literature. However, in the present
work, we show that WDM, modified gravity and coupled CDM form a trinity,
namely, they are indistinguishable by using the cosmological observations of
both cosmic expansion history and growth history. Therefore, to break this
degeneracy, the other complementary probes beyond the ones of cosmic expansion
history and growth history are required.
