The expansion of the universe in the aftermath of the Big Bang (the prevailing cosmological model for the birth of our universe) was triggered by antigravity. Thus, there could have been several Big Bangs even before our universe was created; there could be many more in the future.
In other words, the universe's expansion could continue forever and new universes could be created in the process of constantly repeated expansion of the universe. According to the Big Bang theory, space is evenly filled with an incredible amount of energy, which triggers rapid spatial expansion. During rapid spatial expansion, energy is emitted in all directions in forms similar to sparks of static electricity and rapidly turns into minute particles. This is how a new universe is created according to the Big Bang theory.
Meanwhile, except for newly created universes, the remaining space could be still filled with energy and expanding at incredible speeds. The more spatial expansions, the bigger the amount of energy released and the more Big Bangs. Eventually, more universes could be newly created. This means that the expansion of the universe could never stop. In other words, the permanent expansion of the universe could continuously create new universes, as well as parallel ones.
Yet, there is one caveat to the parallel universe theory: there is no way to detect the existence of a parallel universe. The parallel universe theory, however, has the backing of two completely unrelated research fields: the theory of dark energy and superstring energy. The former takes a macroscopic view in explaining the principle of the universe, as opposed to the latter. Dark energy that exists in space makes all the galaxies repel each other and expands at an ever increasing rate. But astronomers’ measurements of the amount of actually existing dark energy were much smaller than previously projected. The discrepancy between the theoretical value of dark energy and astronomers’ measurements is one of the biggest mysteries scientists are facing.
However, when we simply suppose that we are living in a multiverse, the mystery may not be so mysterious. If our universe is just part of a multiverse and each universe has different amounts of dark energy, the less-than-expected amounts of dark energy would not be so odd. Universes that have less dark energy than our universe does would shrink. On the other hand, those with greater dark energy than we do would expand too fast, stripping matter of opportunities to agglomerate together to form either stars or planets.