Dark energy, a mysterious substance thought to be responsible for accelerating the expansion of the Universe, is real, European astronomers say. Writing in the Monthly Notices of the Royal Astronomical Society, researchers at the University of Portsmouth and LMU University Munich say their two-year study puts the possibility of the existence of dark energy at 99.996 percent. \"Dark energy is one of the great scientific mysteries of our time, so it isn\'t surprising that so many researchers question its existence,\" Portsmouth astronomer Bob Nichol said. \"But with our new work we\'re more confident than ever that this exotic component of the Universe is real -- even if we still have no idea what it consists of.\" Dark energy is thought to make up 73 percent of the content of the universe, creating a repulsive force working against gravity and accelerating its expansion. Astronomers have analyzed the cosmic microwave background, the radiation of the residual heat of the Big Bang, to test the theory light from this radiation would become slightly bluer as it passed through the gravitational fields of lumps of matter, suggesting the presence of dark energy on the assumption its existence would lead to a counterintuitive effect where the cosmic microwave background photons would gain energy rather than lose it as they traveled through the gravitational field. The astronomers conducting the new study conclude there is a 99.996 percent chance dark energy is responsible for the hotter parts of the cosmic microwave background maps. \"The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works,\" study lead author Tommaso Giannantonio said.