  The appearance of a $3.5$ keV photon line in the X-ray spectra of Andromeda
galaxy and other galaxy clusters including the Perseus galaxy can be
interpreted as the signal of a $7$ keV sterile neutrino dark matter candidate
as one of the plausible explanations. We present here a novel framework of
extended seesaw mechanism where the lightest sterile neutrino acts as a Dark
Matter candidate and its decay into a photon and an ordinary neutrino accounts
for the excess 3.5 keV X-ray line. For implementing the idea, we add two
different types of neutral fermion singlets ($S_L$, $N_R$) to the SM light
neutrino $\nu_L$ so that the light neutrino mass is governed by generic linear
seesaw formula. We discuss the complementarity constraints on the model
parameters including sterile-active neutrino mixing angle within the extended
seesaw framework. Favorably, this framework can be embedded in a $SO(10)$ grand
unified theory.
