Like the Many-Worlds theory, string theory demonstrates that parallel universes exist. According to the theory, our own universe is like a bubble that exists alongside similar parallel universes. Unlike the Many-Worlds theory, string theory supposes that these universes can come into contact with one another.

No parallel universe has ever been found. The theory that supports parallel universes is basically the following: We have no idea why certain variables that govern the laws of physics would not permit life if they were changed even slightly.

Participatory Anthropic Principle Wheeler speculated that reality is created by observers in the universe. "How does something arise from nothing?", he asked about the existence of space and time. He also coined the term "Participatory Anthropic Principle" (PAP), a version of a Strong Anthropic Principle.

Modern proponents of one or more of the multiverse hypotheses include Lee Smolin, Don Page, Brian Greene, Max Tegmark, Alan Guth, Andrei Linde, Michio Kaku, David Deutsch, Leonard Susskind, Alexander Vilenkin, Yasunori Nomura, Raj Pathria, Laura Mersini-Houghton, Neil deGrasse Tyson, Sean Carroll and Stephen Hawking.