[ks]

I've thought about that too. What if there is a civilization out there that are millions of years ahead of us. What can't they do?

We think in human terms, but they might not have the need to spread to every planet. But they may want to stop other civilizations, because they can become threats to them. Perhaps they just monitor the galaxy and stops other civilizations from spreading too far?

That could also explain why UFO observations have increased so much after WWII. When they noticed that we could make nuclear bombs, we were put on the "no fly list" :-)