The solution to distance isn’t energy; it’s time. Even at speeds much lower than the speed of light, a civilization could have spread across or explored the Milky Way in a few millions of years. These needn’t have been biological creatures. Von Neumann probes, “Genesis” machines, or just flinging rocks with sensors on them to tour the stars would do the trick.
What would motivate a civilization to undertake such an experiment? A probe a 50,000 light years away would take 50,000 years to send back any meaningful information. Unless alien life-spans are significantly longer than humans, I would think this would probably not be funded by their science department.
Von Neumann probes would be great at asteroid mining. If you just let the probes keep going rather than limiting their distance, they'd cover the 50k light years easily. Not much overhead on the civilization.
A probe of the sophistication of a Von Newman machine, that can reproduce, would be the alien in itself. A network of such probes would be a civilization.
