  The $\Lambda$CDM framework offers a remarkably good description of our
universe with a very small number of free parameters, which can be determined
with high accuracy from currently available data. However, this does not mean
that the associated physical quantities, such as the curvature of the universe,
have been directly measured. Similarly, general relativity is assumed, but not
tested. Testing the relevance of general relativity for cosmology at the
background level includes a verification of the relation between its energy
contents and the curvature of space. Using an extended Newtonian formulation,
we propose an approach where this relation can be tested. Using the recent
measurements on cosmic microwave background, baryonic acoustic oscillations and
the supernova Hubble diagram, we show that the prediction of general relativity
is well verified in the framework of standard $\Lambda$CDM assumptions, i.e. an
energy content only composed of matter and dark energy, in the form of a
cosmological constant or equivalently a vacuum contribution.
  However, the actual equation of state of dark fluids cannot be directly
obtained from cosmological observations. We found that relaxing the equation of
state of dark energy opens a large region of possibilities, revealing a new
type of degeneracy between the curvature and the total energy content of the
universe.
