Universe, in astronomy, all matter and energy whose existence has been, will be, or could be made known to us. It includes all the stars and all their radiations to the farthest reaches of space and of time. The known universethe part that can be detected with optical and radio telescopesextends out in all directions from the sun and earth for a distance of about 10 billion light-years. According to theories based on current observations, the universe is at least 10 billion and possibly 20 billion years old. (The earth was formed about 4 1/2 billion years ago.)

Cosmology, the study of the universe as a whole, attempts to provide an overall picture of the nature and operation of the physical universe. According to many cosmologists, the universe extends in all directions from earth to unknown distances beyond the present limits of observation. Some cosmologists believe the universe has no limits in space or time. Out to the limits of observation, all galaxies seem to be traveling away from the earth and from each other at ever increasing speeds; this provides the basis for the concept that the entire universe, as well as the known part of the universe, is expanding.