What is a black hole?

It sounds like a theory plucked from the page of a science fiction novel, but according to Enrique Gaztañaga from the University of Portsmouth, our entire Universe could be trapped inside a black hole. It's a mind-blowing theory, but it could help us better understand the fundamental nature of our reality.

But how is it even possible for us to be inside a black hole? Will we ever know for sure? And what could lie beyond its boundaries.

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