It was a profound shift to our understanding of the cosmos when 25 years ago astronomers identified that the universe is not only expanding but accelerating away from us. A new force was needed to account for this and dark energy was the solution. But critics claim there is a risk that dark energy is nothing more than an empty hypothesis, since two decades on and we still have no explanation for it and no independent confirmation of its existence. Yet it supposedly makes up 68% of the universe.  

Do we need to get rid of dark energy in favour of an alternative, such as changing Einstein's theory of relativity, or modifying gravity? Is the standard cosmological model of the Big Bang and cosmic inflation itself at stake and might the universe be very different than we currently suppose? Or is the dark energy hypothesis the best hope we have for making sense of the deep mysteries of cosmology?