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Not an astrophysicist (or any other type of trained scientist)....nor do I play one on TV. Just an interested bystander of the universe. I hate to resort to Wikipedia (yeah, it is flawed ) but: "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. While special relativity constrains objects in the Universe from moving faster than light with respect to each other when they are in a local, dynamical relationship, it places no theoretical constraint on the relative motion between two objects that are globally separated and out of causal contact. It is thus possible for two objects to become separated in space by more than the distance light could have travelled, which means that, if the expansion remains constant, the two objects will never come into causal contact. For example, galaxies that are more than approximately 4.5 gigaparsecs (14.7 billion light-years) away from us are expanding away from us faster than light. We can still see such objects because the Universe in the past was expanding more slowly than it is today, so the ancient light being received from these objects is still able to reach us, though if the expansion continues unabated, there will never come a time that we will see the light from such objects being produced today (on a so-called "space-like slice of spacetime") and vice versa because space itself is expanding between Earth and the source faster than any light can be exchanged."