According to measurements, the Universe's expansion rate was decelerating until about 5 billion years ago due to the gravitational attraction of the matter content of the Universe, after which time the expansion began accelerating. In order to explain the acceleration physicists have postulated the existence of dark energy which appears in the simplest theoretical models as a cosmological constant. According to the simplest extrapolation of the currently-favored cosmological model (known as "ΛCDM"), this acceleration becomes more dominant into the future.
