Everything that physically exists, including the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. The universe (or cosmos) is usually considered to have begun about 13.7 billion years ago in a gravitational singulary commonly known as the Big Bang, and has been expanding ever since. Some have speculated that this universe is just one of many disconnected universes, which are collectively denoted as the multiverse.