Confronting Galactic and Extragalactic $\gamma$-ray observed by
  Fermi-LAT with Annihilating Dark Matter in Inert Higgs Doublet Model
  In this thorough study we focus on the indirect detection of Dark Matter (DM)
through the confrontation of unexplained galactic and extragalactic
$\gamma$-ray signatures for a low mass DM model. For this, we consider a simple
Higgs-portal DM model, namely, the inert Higgs doublet model (IHDM) where the
Standard Model is extended with an additional complex SU(2)$_L$ doublet scalar.
The stability of the DM candidate in this model, i.e., the lightest neutral
scalar component of the extra doublet, is ensured by imposing discrete
$\mathbb{Z}_2$ symmetry. The reduced-$\chi^2$ analysis with the theoretical,
experimental and observational constraints suggests the best-fit value of DM
mass in this model to be $\sim$ 63.5 GeV. We analyse the anomalous GeV
$\gamma$-ray excess from Galactic Centre in light of the best-fit IHDM
parameters. We further check the consistency of the best-fit IHDM parameters
with the Fermi-LAT obtained limits on photon flux for 18 Milky Way dwarf
spheroidal satellite galaxies (dSphs) known to be mostly dominated by DM. Also
since the $\gamma$-ray signal from DM annihilation is assumed to be embedded
within the extragalactic $\gamma$-ray background (EGB), the theoretical
calculations of photon flux for the best-fit parameter point in the IHDM
framework are compared with the Fermi-LAT results for diffuse and isotropic EGB
for different extragalactic and astrophysical background parametrisations. We
show that the low mass DM in IHDM framework can satisfactorily confront all the
observed continuum $\gamma$-ray fluxes originated from galactic as well as
extragalactic sources. The extensive analysis performed in this work is valid
for any Higgs-portal model with DM mass in the ballpark of that considered in
this work.
