Once upon a time, maps were full of blank space, vast regions that had never been explored — at least not by anyone mapmakers knew about. These unknown regions were an invitation to mapmakers’ imagination. As one legendary mapmaker wrote, “Here be dragons.”

Explorers gradually filled in the map of the world, as our most recent robotic space explorers have filled in our maps of Pluto and the rings of Saturn. But out at the far frontiers of theoretical physics, we have found tantalizing hints of more unexplored regions than we could ever have imagined.

Welcome to the multiverse theory.

When Things Get Strange — and Stranger

Thanks to the surprise Netflix hit “Stranger Things” — a TV show about a telepathic girl and her friends battling creatures from a parallel universe called the Upside Down — multiverse theory is gaining major traction in pop culture. But the ideas behind multiverse theory have been unfolding for decades, and along multiple threads.

Some threads have developed from the sciences of astrophysics and cosmology, while other, even older, threads have emerged from science fiction and philosophical speculation.

The term “multiverse” describes the multitude of possible universes. As often happens along the frontiers, there is some inconsistency in language. As Yasmin Tayag notes at Inverse, to cosmologists, multiverse describes the sum total of all possible universes, while in more popular language it has come to mean any universe we might encounter other than our own.

By the middle of last century, astronomers determined that the universe is expanding. They soon discovered that this expansion was not a smooth process. Evidence pointed toward an early period of fantastically rapid expansion which astrophysicists dubbed inflation.

The concept of inflation is now generally accepted by physicists, but inflation theory also has a host of subtle implications that are still being explored. If the expansion of the universe was almost unimaginably fast, the result might also be unimaginably vast — a scale of size that would make the mere billions of light-years we can observe look insignificant in comparison.

Indeed, distance measurements such as light-years might not even be meaningful, since the inflating universe may have split into innumerable “bubbles” — each one a universe in its own right, with its own laws of physics and unfolding reality. (A key word is may: all of this remains under study by cosmologists, with much disagreement among them about the implications.) “Our universe is just one of these bubbles,” Scientific American speculates.

Beyond the Doors of Imagination

The possibilities opened up by multiverse theory, of multiple and parallel universes, have long been a topic of speculation by philosophers. What is new is the potential for some sort of experimental confirmation or at least indirect evidence that they exist.

And just as philosophers have long speculated about multiverse-like ideas, so have storytellers. One theme, familiar from the classic original Star Trek episode “Mirror Mirror,” is a universe that is almost like our own, but with a literally cosmic twist — the crew of the USS Enterprise are bad guys in the mirror universe.

Alternate history has become an entire popular genre with elements of multiverse theory. For example, the “Back to the Future” films played with the ways that different outcomes of past events could spin off universes in which the future of those events played out in different ways. (Fun fact — one of the films was only off by a single year in guessing when the Chicago Cubs would win a World Series.)

Understandably, the pop culture version of the multiverse is not quite the same as the astrophysicists’ version, and stories like “Stranger Things” and its earlier cousins freely intermix ideas that come out of physics with others that still belong to metaphysics.

But if we here at Now. were to make a guess — and we will — whatever ultimately comes out of multiverse theory as it develops will probably be stranger and more wonderful than anything that our present-day imaginations can come up with. On the frontiers of science and technology, that is usually how it is.