New observational constraints on the cosmic matter density \Omega _ { m } and an effectively redshift-independent equation of state parameter w _ { x } of the dark energy are obtained while simultaneously testing the strong and null energy conditions of general relativity on macroscopic scales .
The combination of REFLEX X-ray cluster and type-Ia supernova data shows that for a flat Universe the strong energy condition might presently be violated whereas the null energy condition seems to be fulfilled .
This provides another observational argument for the present accelerated cosmic expansion and the absence of exotic physical phenomena related to a broken null energy condition .
The marginalization of the likelihood distributions is performed in a manner to include a large fraction of the recently discussed possible systematic errors involved in the application of X-ray clusters as cosmological probes .
This yields for a flat Universe , \Omega _ { m } = 0.29 ^ { +0.08 } _ { -0.12 } and w _ { x } = -0.95 ^ { +0.30 } _ { -0.35 } ( 1 \sigma errors without cosmic variance ) .
The scatter in the different analyses indicates a quite robust result around w _ { x } = -1 , leaving little room for the introduction of new energy components described by quintessence-like models or phantom energy .
The most natural interpretation of the data is a positive cosmological constant with w _ { x } = -1 or something like it .