The Multiverse
As an irrelevant aside, contrary to the the narrator's claim in What We Still Don't Know, Astronomer Royal Sir Martin Rees certainly did not coin the term "multiverse" (the idea that what we call the whole Universe is just one among many, perhaps each with its own properties); it was used in superhero comics in the 70s and 80s, and probably in science fiction well before then.
Subscribe to my feed.