  The most important problem in fundamental physics is the description of the
contents of the Universe. Today, we know that 95% thereof is totally unknown.
Two thirds of that amount is the mysterious Dark Energy described in an
interesting and important review. We briefly extend here the ideas contained in
that review including the more general Dark Sector, that is, Dark Matter and
Dark Energy, eventually composing a new physical Sector. Understanding the Dark
Sector with precision is paramount for us to be able to understand all the
other cosmological parameters comprehensively as modifications of the modelling
could lead to potential biases of inferred parameters of the model, such as
measurements of the Hubble constant and distance indicators such as the Baryon
Acoustic Oscillations. We discuss several modern methods of observation that
can disentangle the different possible descriptions of the Dark Sector. The
possible application of some theoretical developments are also included in this
paper as well as a more thorough evaluation of new observational techniques at
lower frequencies and gravitational waves.
