The universe has been known to be expanding since 1929, when Edwin Hubble measured the speed at which distant galaxies appear to be receding from Earth's position in space. In the early 21st century, astronomers measured the rate of expansion and discovered that it is increasing.
An increasing rate of expansion means that, contrary to what might be expected from classical models, the universe contains insufficient matter to slow or reverse the expansion. In the present epoch, the rate of expansion has been measured at 42 miles per second per 1 million light-years. This means that distant galaxies are receding much faster than nearby galaxies, and that objects near the universal horizon, which is the farthest an Earthbound observer can theoretically observe, will eventually disappear over the edge and will no longer be visible from Earth.
Astrophysicists have modeled the expansion using a concept called dark energy, which describes the poorly understood force that is overcoming gravity and driving the expansion ever faster. The nature of dark energy, and even whether it is a form of energy at all, is unknown. Given enough time, the universe is destined to expand until nothing remains in existence but a diffuse soup of radiation in a near-perfect vacuum.