BSL Astronomy Glossary - dark energy - definition
Definition: After the Big Bang, 13.8 billion years ago, the universe started to expand. Scientists have realised that this expansion is not just due to the force of the original explosion, as they have discovered that the rate of expansion is accelerating. Galaxies are moving away from each other with increasing speed and scientists do not yet know what is causing this. Dark energy is the name they have given to the force that causes this accelerated expansion, although it may not be dark and may not even be energy, as we understand it today.