Dark energy is the name given to a hypothetical substance in our universe which has the peculiarity of having negative pressure. It could be the cause of the acceleration in the expansion of the universe, shown experimentally in 1998.
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A diagram showing the relative contribution of dark energy in the universe. Estimates vary, but a value slightly higher than 70% is usually given. Normal matter only contributes about 4% to the energy content of the observable universe, the rest being dark matter. © Nasa CXC M.Weiss