  We comment on cases in the history of Astronomy, which may shed some light on
the current established but enigmatic concordance model of Cosmology. Should
the model be understood by adding new entities such as Dark Matter and Dark
Energy, or by modifying the underlying theory? For example, the prediction and
discovery of planet Neptune can be regarded as analogous to finding a dark
component; while explaining the anomalous perihelion precession of Mercury by
General Relativity can be taken as analogous to the possibility that modified
gravity is an alternative to dark components of the universe. In this paper, we
revise this analogy coming from the history of astronomy with an eye to
illustrating some of the similarities and differences between the two cases.
