  The mysterious `dark energy' needed to explain the current observations,
poses a serious confrontation between fundamental physics and cosmology. The
present crisis may be an outcome of the (so far untested) prediction of the
general theory of relativity that the pressure of the matter source also
gravitates. In this view, a theoretical analysis reveals some surprising
inconsistencies and paradoxes faced by the energy-stress tensor (in the
presence of pressure) which is used to model the matter content of the
universe, including dark energy.
