Researchers commonly attribute the acceleration to some mysterious entity called dark energy, but there is little physics to back up these fine words. The only thing that is becoming clear is that at the largest observable distances, gravity behaves in a rather strange way, turning into a repulsive force. The laws of physics believe that gravity is generated by matter and energy, so they attribute a strange sort of gravity to a strange sort of matter or energy. That is the rationale for dark energy.
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