  The current standard model of cosmology - the {\ensuremath{\Lambda}}CDM model
- is appropriately named after its controversial foreign ingredients: a
cosmological constant ({\ensuremath{\Lambda}}) that accounts for the recent
accelerated expansion of the Universe and cold dark matter needed to explain
the formation and dynamics of large scale structures. Together, these form the
dark sector, whose nature remains a mystery. After 25 years of withstanding
confirmation and support for the {\ensuremath{\Lambda}}CDM model, enough to
bypass some of its unclear theoretical issues, this paradigm is facing its
biggest crisis yet. The rapid advent of technology has brought cosmology to an
unprecedented observational era, with increased technical precision and the
emergence of independent measures, including probes of phenomena that were
thought impossible to detect or even exist, such as the gravitational ripples
that propagate in the spacetime. However, such precision has unveiled cracks in
the porcelain of {\ensuremath{\Lambda}}CDM, with pieces that seem glued
together and difficult to reconcile. Particularly worrying is the apparent lack
of compatibility between measurements of the Universe's present expansion rate
based on local measurements and those based on phenomena that occurred far in
the early Universe and that can only be translated into present quantities
through physical propagation under a cosmological model. In this dissertation,
we delve into extensions to the standard model that consider alternatives to
the mysterious nature of the dark sector and any possible new interactions
therein. We analyse these alternative models, hoping to identify measurable
observational signatures of extra degrees of freedom in the dark sector.
