We think of our Universe as everything there ever was, is, and will be.

But according to some researchers, there might not just be many universes, but an infinite number of them. This notion of multiple universes – sometimes dubbed the multiverse for short – isn't some crazy idea concocted by bored physicists. While the science is undeniably speculative, they emerge from fairly well-grounded theories. And recent discoveries have made headlines for supporting the idea. There's a lot that physicists don't yet know, but the existence of the multiverse is possible, and some might say probable.

Yet if there really were other realms outside our own, how would we even know? Will we ever be able to detect another universe?

"I do think you can find smoking-gun evidence for things outside of [our Universe]," says Anthony Aguirre, a physicist at the University of California, Santa Cruz. After all, not being able to directly see or hold an atom didn't preclude physicists from confirming their existence.

Perhaps the most plausible type of multiverse is a natural consequence of a theory called inflation. The Universe expanded rapidly after the big bang, and it continues to do so today. But according to inflation, the Universe grew exponentially fast in the first moments of its existence, an instant of faster-than-light expansion.

Physicist Alan Guth proposed this radical idea in 1980 to explain several features of the Universe: why it looks the same in every direction, for instance. Since then, physicists such as Andrei Linde have further developed the theory, which has been supported by observations of the cosmic microwave background (CMB) – the leftover glow of the Big Bang that fills the sky.