There's always the theoretical extension of a black hole into a corresponding quasar on the other end.
The thing with this, is that black holes would be wormholes not just through space but through time; specifically back
in time. Quasars are extremely distant, a.k.a. extremely old objects, that are relics of a bygone era.
When the universe was younger, smaller, hotter, and denser, galaxies and super-sized stars were piling up the mass through gravity, and these central galactic black holes were consuming matter voraciously. The combustion of material accreting about these monsters produces these relativistic jets of blazing energy and mass in opposite directions, from the 'poles' of a mega black hole. When they are aimed in our direction we see something resembling a very brilliant star, so bright that it's visible halfway across the observable universe.
Quasars are not 'white holes' or the end point of a black hole that's been theorized, the fact that there are no (relatively) nearby quasars in the local universe is a good indicator that they've all gone away from the universe and cooled down, and thus the relics we see today are just objects billions of years old because the light from them has been traveling all that time to us.