Isn “dark energy” just like the older concept of the “ether”?
No; in fact, it’s just the opposite. The ether was supposed to be an invisible substance that determined the rest frame of the universe. It was expected by theorists, but eventually abandoned when experimenters could not find any evidence for it (and Einstein figured out that it wasn’t necessary). Dark energy, meanwhile, was not at all expected by most working cosmologists; we need it to explain observed facts, like the acceleration of the universe and the mismatch between matter and total energy. And the dark energy appears the same to all observers, so there’s no sense in which it determines a rest frame.
