The nature of dark energy is the biggest problem in cosmology. But the answer may not be very different to what most cosmologists assume. General relativity is not a complete theory. It leaves many important questions unanswered, including the nature of gravitational energy. This is directly relevant to an important observational fact: the Universe is a very inhomogeneous cosmic web on the small scales on which general relativity is actually tested. How one fits one geometry inside another to arrive…