  Is Dark Energy justified as an alternative to the cosmological constant
$\Lambda$ in order to explain the acceleration of the cosmic expansion ? It
turns out that a straightforward dimensional analysis of Einstein equation
provides us with clear evidences that the geometrical nature of $\Lambda$ is
the only viable source to this phenomenon, in addition of the application of
Ockham's razor principle. This contribution is primarily a review of the main
stream in the interpretation of $\Lambda$ because it is at the origin of such a
research program.
