The universe can be defined as everything there is, including quarks, electrons, photons, atoms, molecules, cells, animals, planets, stars, galaxies, galaxy clusters, superclusters, filaments, voids, and more things yet to be discovered. Researchers however refer to the universe as the observable universe: the spherical region surrounding Earth that contains matter that our current telescopes, whether Earth based or not, and our current probes, can detect, because light has had time to reach us since the big bang. This leaves open the possibility of other universes, or parallel universes, which exist beyond our observable universe. The idea of parallel universes is not without controversy, but as years passed, the scientific community has come to be more open towards this idea. Physicists such as Max Tegmark argue that parallel universes are not a theory, but a part of theories that have been tested and have passed those tests time and time again. Max Tegmark argues that because those theories have passed all tests so far, we have no basis for rejecting the theory, which also encompasses those features we cannot currently test, namely parallel universes. However, other researchers argue that because parallel universes are not testable, they are not scientific ideas.
For more information, see:
https://science.nasa.gov/observable-universe
https://www.space.com/32728-parallel-universes.html
If you want to know more about different types of parallel universes, see Max Tegmark's website:
