However, scientists say it is unlikely anyone passing through one of the gateways would survive.
If they did pass through, they would be "spaghettified" - which means being stretched out like a strand of pasta - by the black hole's immense gravity.
Once on the other side of the "door" the traveller would be compacted back to normal size, but could not count on being restored back to life.
Black holes are places where matter has been squashed to such a density by gravity that the normal laws of physics break down.
The new theory rejects the view that at the centre of a black hole spacetime curves to an infinite point known as a "singularity" and all matter is destroyed.
Instead, it proposes that the heart of the simplest type of electrically charged, non-rotating black hole, is a very small spherical surface. This acts as a "wormhole" - a doorway or tunnel through the fabric of spacetime of the kind seen in countless sci-fi stories.
In the movie Interstellar, a team of astronauts travel through a wormhole in search of a new home for humanity.
Dr Gonzalo Olmo, from the University of Valencia in Spain, said: "Our theory naturally resolves several problems in the interpretation of electrically-charged black holes.
"In the first instance, we resolve the problem of the singularity, since there is a door at the centre of the black hole, the wormhole, through which space and time can continue."
The wormhole predicted by the scientists' equations is smaller than an atomic nucleus, but gets bigger as more electrical charge is stored in the black hole.
A hypothetical traveller entering the black hole could be stretched thin enough to fit through the wormhole, like a strand of cotton threaded through the eye of a needle.