[Damorian]

Not OP, but I fully agree with them. Incoming ballpark figures I and hand waving. If, today, with our current technology, we decided to explore/colonize every star and planet in our galaxy, launching 3 ships per day that travel at single digits percent of c, we could investigate every planet in our galaxy in about a billion years. That's pretty much worse case technological scenario, because at some point in the next thousand or million years we'll have even more incredible technology to build more and fly faster. Probes that can constantly accelerate at 1g will reach other galaxies on the order of millions of years. If they self replicate, or have pioneers that can rebuild civilization on the order of a few thousand years and continue exploring, we're still within that 1 billion year limit to colonize at worst the entire galaxy, and realistically the entire universe. Earth has been around for 5 billion years, and the greater universe for 15. Humans have only existed for 10,000 years. ET should have been here long before we even came into existence.

[solidasparagus]

We only have a few thousand years of reliable history and in many places, far less than that. It's perfectly reasonable in your scenario that ET was here long before we were, missed us by a couple of million years and left.

If you look at the history of human exploration, discovering new lands has not led to immediate settling or the establishment of long-term relationships with the locals.

[StreamBright]

Assuming they do not have a prime directive.