  The nature of dark matter (DM) and how it might interact with the particles
of the Standard Model (SM) is an ever-growing mystery. It is possible that the
existence of new `dark sector' forces, yet undiscovered, are the key to solving
this fundamental problem, and one might hope that in the future such forces
might even be `unified' with the ones we already know in some UV-complete
framework. In this paper, following a bottom-up approach, we attempt to take
the first steps in the construction of such a framework. The much-discussed
possibility of the kinetic mixing (KM) of the `dark photon' with the
hypercharge gauge boson of the SM via loops of portal matter (PM) fields,
charged in both sectors, offers an attractive starting point for these efforts.
Given the anticipated finite strength of the KM in a UV-complete theory, the
absence of anomalies, and the lifetime constraints on the PM fields arising
from CMB and nucleosynthesis constraints, PM must behave as vector-like copies
of the known SM fermion fields, such as those which appear naturally in
$E_6$-type models. Within such a setup, the SM and their corresponding partner
PM fields would be related by a new $SU(2)_I$ gauge symmetry. With this
observation as a springboard, we construct a generalization of these ideas
where $SU(2)_I$ is augmented by an additional $U(1)_{I_Y}$ factor so that the
light dark photon is the result of a symmetry breaking analogous to the SM,
\ie, $SU(2)_I\times U(1)_{I_Y}\to U(1)_D$, but with $U(1)_D$ now also broken at
the $\lesssim$ GeV scale. While SM fields are $U(1)_D$ singlets, as in the
conventional dark photon approach, they transform nontrivially under the full
$SU(2)_I\times U(1)_{I_Y}$ gauge group. This approach leads to numerous
interesting signatures, both at low energies and at colliders, and can be
viewed as an initial step in the construction of a more UV-complete framework.

High Energy Physics - Phenomenology