[philipkglass]

The hypothetical aliens don't need to be mortal biological creatures. An advanced probe "crewed" by artificial intelligence could take thousands of years to send copies of itself from one star to the next using propulsion concepts from the 1960s:

https://en.wikipedia.org/wiki/Bracewell_probe

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

They could have have arrived in the solar system 100 million years ago, just running maintenance routines and sending observations back home ever since. They don't need to know anything about human culture to stay largely unobserved, any more than human bird watchers need to know bird culture.

I don't think that this is particularly likely but known physics permits alien visitors if you assume that they are stealthy (not acquisitive) and very patient.

[phs318u]

To be honest, the hardest thing for me to believe with this hypothesis is the possibility of a society that can not only survive for 100 million years, but maintain an interest in a subject for that long.

[HKH2]

Well, people will continue to wonder if any other places in the universe have sentient beings. Not sure why the question will go away when what caused the question is still around.