Proponents of "dark energy" claim that it's existence is proven, even if not a single sample exists. For proof they cite a complex chain of reasoning. Like the Emperor's New Clothes, only the truly enlightened can understand the evidence. Everyone else, even other scientists, can only quote the meme that DE must exist.
The September 29 issue of NATURE contains this paper:
Gravitational redshift of galaxies in clusters as predicted by general relativity
"The theoretical framework of cosmology is mainly defined by gravity, of which general relativity is the current model. Recent tests of general relativity within the Lambda Cold Dark Matter (ΛCDM) model have found a concordance between predictions and the observations of the growth rate and clustering of the cosmic web. General relativity has not hitherto been tested on cosmological scales independently of the assumptions of the ΛCDM model. Here we report an observation of the gravitational redshift of light coming from galaxies in clusters at the 99 per cent confidence level, based on archival data. Our measurement agrees with the predictions of general relativity and its modification created to explain cosmic acceleration without the need for dark energy..."
Despite the many press releases about "dark energy," doubts continue to grow. While data may fit a universe with DE, the same data also corresponds to no DE at all. Hypotheses of a repulsive "dark energy" has not led to theory, just a divergence of speculations that erode the reputation of science. While there will be no samples of DE, there is evidence of a changing speed of light.