Maybe this is where the aliens have gone

Pair of researchers suggest black holes at center of galaxies might instead be wormholes

What an interesting suggestion:

The supermassive black hole candidates at the center of every normal galaxy might be wormholes created in the early Universe and connecting either two different
regions of our Universe or two different universes in a Multiverse model. Indeed, the origin of these supermassive objects is not well understood, topological non-trivial structures like wormholes are allowed both in general relativity and in alternative theories of gravity, and current observations cannot rule out such a possibility.

A good idea for science fiction, too.