  To trace the particle nature of dark matter (DM) from cosmic microwave
background observations, we study the co-evolution of DM density perturbation
and primordial metric perturbation during DM production. Solving the
perturbative Einstein and Boltzmann equations, we unveil an amplified
perturbative resonance between the DM density perturbation and the scalar modes
of metric perturbation, driven by DM production. Such perturbative resonance
does not affect the tensor modes and suppresses the tensor-to-scalar ratio of
metric perturbation $r$. Using a typical model to specify cosmic reheating, we
obtain a direct relation between DM particle mass $m_\chi$ and $r$, which can
constrain $m_\chi$ in future primordial gravitational wave searches.
