The Universe is the totality of everything that exists, has existed, and ever will exist.[2] Similar meaning is sometimes conveyed with the words cosmos, world, reality, and nature. The Universe includes all of spacetime; the entire contents of outer space; all matter, energy, dark matter, and dark energy; all galaxies, stars, and planets; all humans and every living thing; all molecules, atoms, subatomic particles, photons, and their wave dualities; all physical constants, physical laws and fundamental interactions. The Universe can even be understood to encompass all of mathematics, all concepts and ideas, and all thoughts and emotions.[3][4]