An interesting theory, in need of a *lot* of evidence to back it up:

Scientists think there may be a wormhole in the center of our galaxy

Or go straight to the Arxiv paper:

Distinguishing black holes and wormholes with orbiting hot spots

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. In a few years, the VLTI instrument GRAVITY will have the capability to image blobs of plasma orbiting near the innermost stable circular orbit of SgrA∗, the supermassive black hole candidate in the Milky Way. The secondary image of a hot spot orbiting around a wormhole is substantially different from the one of a hot spot around a black hole, because the photon capture sphere of the wormhole is much smaller, and its detection could thus test if the center of our Galaxy harbors a wormhole rather then a black hole.

It would appear to be largely hypothetical handwaving at this point, but it’s an interesting idea. At the center of many/most galaxies, there might be a very large wormhole that leads *elsewhere.* Perhaps to the other side of the universe, perhaps to a completely different universe… and perhaps to different points in time. Of course, traveling the 30,000 or so lightyears to get to the center of the galaxy might be a bit of a schlep. But if you assume arbitrarily advanced transport technology, and if the wormholes are sufficiently common and do in fact lead to other places in our universe and other times, one could posit a far distant future where a starship sets out from Earth, proceeds to 99.9999…% lightspeed so that the travel time as experienced by the crew is just a few days, goes through the wormhole and pops out in a galaxy say 300 million lightyears away… and 400 million years in the past. The starship then sets out back for the home galaxy, again at 99.999999…% lightspeed, and gets home 100 million  years in the past. It then finds Earth, picks up some dinosaurs, and goes on a tour of the galaxy, returning to Earth every two or three million years, picking up new critters, watching the K-T impactor, and finally stopping at Earth at, oh, 100,000 BCE, where it then drops off a bunch of probes that watch the world closely from then on. Every now and then it drops organic androids to interact with the locals, and examines all of human history up close. When it finally gets to its own original time period – say, 3,000 CE – it pops out of hiding five minutes after it left, and reveals all its treasures.

Or… not.