  In this paper we review a part of the approaches that have been considered to
explain the extraordinary discovery of the late time acceleration of the
Universe. We discuss the arguments that have led physicists and astronomers to
accept dark energy as the current preferable candidate to explain the
acceleration. We highlight the problems and the attempts to overcome the
difficulties related to such a component. We also consider alternative theories
capable of explaining the acceleration of the Universe, such as modification of
gravity. We compare the two approaches and point out the observational
consequences, reaching the sad but foresightful conclusion that we will not be
able to distinguish between a Universe filled by dark energy or a Universe
where gravity is different from General Relativity. We review the present
observations and discuss the future experiments that will help us to learn more
about our Universe. This is not intended to be a complete list of all the dark
energy models but this paper should be seen as a review on the phenomena
responsible for the acceleration.
  Moreover, in a landscape of hardly compelling theories, it is an important
task to build simple measurable parameters useful for future experiments that
will help us to understand more about the evolution of the Universe.
