The Universe is all of time and space and its contents.[9][10][11][12] It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. The observable universe is about 28 billion parsecs (91 billion light-years) in diameter.[3] The size of the entire Universe is unknown, but there are many hypotheses about the composition and evolution of the Universe.[13]