Cosmology is the study of the organization and evolution of the universe. Measurements of distant supernovae show that when the universe was about half its current age, dark energy began to dominate the rate of expansion and caused it to speed up.

After allowing for these effects, astronomers estimate that all of the matter within the observable universe was concentrated in an extremely small volume 13.8 billion years ago, a time we call the Big Bang The universe is expanding, and this is one of the key observational starting points for modern cosmological theories.

Big Bang — the theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy). dark energy — the energy that is causing the expansion of the universe to accelerate; its existence is inferred from observations of distant supernovae. universe — expanding, and this is one of the key observational starting points for modern cosmological theories. cosmology — the study of the organization and evolution of the universe.