“David Wiltshire, a physicist at the University of Canterbury in New Zealand and a visitor to Barbour’s College Farm, thinks the reason dark energy is so mysterious is that it is an illusion. Wiltshire’s argument is that most physicists essentially ignore one of the major principles at the heart of general relativity: that clocks in different parts of the universe can run at different rates. Einstein held that there is no such thing as universal time and that matter affects the rate at which clocks tick, such that time slows near massive objects. Accordingly, Wiltshire notes, the flow of time near galaxies could be slower than the flow of time in empty space. “In a truly relativistic view, the age of the universe differs from place to place,” he says. “In empty space, over 18 billion years have elapsed since the Big Bang, but within galaxies only about 15 billion years have passed.” (Because Wiltshire starts from a separate set of physical assumptions, his numbers are different from the now canonical 13.7 billion years for the age of the universe.)” (source)

The previous quote is from an article I quoted quite a bit (in “What Time Is Now?”), but I didn’t want to get into much detail about dark energy as that was not the goal :)