  According to the Standard Model (SM), we expect to find a proton for each
decaying neutron. However, the experiments counting the number of decayed
neutrons and produced protons have a disagreement. This discrepancy suggests
that neutrons might have an exotic decay to a Dark Sector (DS). In this paper,
we explore a scenario where neutrons decay to a dark Dirac fermion $\chi$ and a
non-abelian dark gauge boson $W'$. We discuss the cosmological implications of
this scenario assuming DS particles are produced via freeze-in. In our proposed
scenario, DS has three portals with the SM sector: (1) the fermion portal
coming from the mixing of the neutron with $\chi$, (2) a scalar portal, and (3)
a non-renormalizable kinetic mixing between photon and dark gauge bosons which
induces a vector portal between the two sectors. We show that neither the
fermion portal nor the scalar portal should contribute to the production of the
particles in the early universe. Specifically, we argue that the maximum
temperature of the universe must be low enough to prevent the production of
$\chi$ in the early universe. In this paper, we rely on the vector portal to
connect the two sectors, and we discuss the phenomenological bounds on the
model. The main constraints come from ensuring the right relic abundance of
dark matter and evading the neutron star bounds. When dark gauge boson is very
light, measurements of the Big Bang Nucleosynthesis impose a stringent
constraint as well.
