  We describe the challenges and pathways when probing inflaton as dark matter
with the stochastic gravitational waves (GWs) signal generated during the
(p)reheating. Such scenarios are of utmost interest when no other interaction
between the visible and dark sectors is present, therefore having no other
detectability prospects. We consider the remnant energy in the coherently
oscillating inflaton's zeroth mode to contribute to the observed relic dark
matter density in the Universe. To fully capture the nonlinear dynamics and the
effects of back-reactions during the oscillation, we resort to full nonlinear
lattice simulation with pseudo-spectral methods to eliminate the differencing
noises. We investigate for models whose behavior during the reheating era is of
$m_{\Phi}^2\Phi^2$ type and find the typical primordial stochastic GWs
backgrounds spectrum from scatterings among highly populated inflaton modes
behaving like matter. We comment on the challenges of constructing such viable
inflationary models such that the inflaton will account for the total dark
matter of the universe while the produced GWs are within the future GWs
detectors such as BBO, DECIGO, PTA, AION-MAGIS, and CE. We also describe the
necessary modifications to the standard perturbative reheating scenario to
prevent the depletion of residual inflaton energy via perturbative decay.
