I refer you to Ruth Durrer's very readable (and short) paper with the same title as above. The main point of the paper is that all evidence for Dark Energy comes from the measurement of luminosity distance as a function of redshift and that energy/matter content of the universe is then inferred from this measurement by equation (3) of the paper (go check it out...). Thus the assumptions that feed into equation (3) also affect our interpretation of the dark energy content of the universe. This equation is, apparently, derived by assuming that the geometry of our universe is given by the Friedmann-Lemaître model, which I assume is a FLRW metric with a specific assumption about the expansion. WARNING: I could be very wrong about that.
In case the paper feeds my love/hate relationship with Dark energy very nicely and reads easily (even if you ignore most of the equations). Seriously: The paper gives an accessible introduction...