The Multiverse Theory
The multiverse theory suggests that there are multiple sets of universes (including the one we currently live on). The multiverse includes everything that exists and can/has exist: space, time, matter, and energy. The theory was suggested in 1895 by an American philosopher and psychologist William James. The many universes within the multiverse are often called parallel universes. There has been plenty of criticism and questioning on this theory. A common asked question is 'how has the existence of other universes even been tested?'. This theory obvious has a lot unanswered questions, making it a not very likely theory.
The Big Crunch Theory
The 'Big Crunch' theory proposes that the universe will eventually stop expanding and start to collapse upon itself during a certain stage of its life. This theory corresponds with the Big Bang Theory. When the Big Crunch takes place the universe would contract, this would happen in about the same amount of time as the big bang. It is impossible to know what would happen after this, as time would stop. Some theorists say they have proven this theory incorrect, and claim the universe will continue to expand forever.