Universe
The Universe is a term often used to refer to all that we know exists. It is sometimes speculated that there are other universes, but quite how they are bounded, what part of reality is delimited as “a universe”, whether we could get to them, whether if we could get to them they would constitute other universes, and whether they matter at all if we can’t get to them are quite different questions.The KTAB World Factbook contains a short and slightly incomplete catalogue of the contents of the Universe, an index of which can be found on the “all pages” page.
Theories
The Ancient Greeks created models which at their culmination speculated that the Earth is surrounded by some great anthropocentric merry-go-round of crystal spheres, epicycling planets and little boxes containing the weather.
Nowadays, we believe that the Universe was created in a great explosion of some quantum-relativistic ball of infinitely dense matter whose physics may never be known.
Both are approximately equally plausible.