You appear to be stuck on the assumption that any aliens don’t know any more about the universe than we do and can only use technologies that we understand. That’s typically the result of anthropomorphism and can limit thinking about unknown unknowns.They would have to be at least 10,000+ years older than us. With said generational ships to get here. I really doubt other beings are much more advanced than we are because of the time it takes to develop the intelligence and ability to travel space. We are a long way off from being able to build a generational ship that is self sustainable for thousands of generations.