  During the evolution of the universe there are at least two epochs during
which electromagnetic waves cannot scan the universe's internal structure
neither bring information to outside observers. The first epoch is when photons
are in local thermodynamic equilibrium with other particles, and the second is
when photon scattering by charged particles is strong. One can call these two
periods of cosmological time as standard unlighted epochs. After the last
scattering surface, photons become relic photons and turn into a source of
information about the universe. Unlighted cosmic epochs can also appear when
one considers non-minimal theories, i.e., theories in which the electromagnetic
field is coupled in an intricate way with the cosmological gravitational field.
By considering a cosmological model where the dark sector, i.e., the dark
energy and dark matter, self-interacts via an Archimedean-type force, and
taking into account a non-minimal coupling theory for the electromagnetic
field, we discuss the appearance of unlighted epochs. In the framework of our
non-minimal theory, a three-parameter non-minimal Einstein-Maxwell model, the
curvature coupling can be formulated in terms of an effective refraction index
n(t). Then, taking advantage of a well-known classical analogy, namely, in a
medium with n^2<0 electromagnetic waves do not propagate and their group
velocity, i.e., energy transfer velocity, has zero value at the boundary of the
corresponding zone, one can search for the unlighted epochs arising in the
interacting dark fluid cosmological model. We study here, both analytically and
numerically, cosmological models admitting unlighted epochs.

General Relativity and Quantum Cosmology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Theory