The evidence is that Dark Energy is responsible for the rate of expansion of the universe. The name makes it sound like a spooky force, but, in fact, it’s the cosmological constant, Λ, that Einstein added to his theory of gravity back in 1917. Einstein regretted it, but we have been forced to put Λ back in the theory to fit the evidence. There is theoretical backing of a sort from quantum theory, which also predicts the presence of a cosmological constant, Λ, but with a value that is far off what we need. Finding a way to unify the predicted value of Λ by quantum theory, and the observed value of Λ from the expanding universe, would be a great discovery. But even the most sophisticated theory is constrained by observational evidence, which always will be imperfect and incomplete, and hence our theories always will be an approximation, never an account of ultimate reality, argues James Peebles. The standard theory for explaining the expansion of the universe …
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