which is where you make an assumption. Science is just a boundary between what we know(or have a good understanding of and evidence) and the unknown......we have no idea of what technology may be possible to enable a species to spread across the galaxy, so we can't really assign any degree of probability to whether they have done, or not.
We do know that we are here, so it is fair to say that there probably are other intelligent species out there, and that maybe the Universe is teeming with intelligent ETs.
Oh, absolutely! I agree with you 100%. We don't know everything, not even close, and we probably never will.
That still doesn't mean we can just make unfounded assumptions about the universe.
What we have to go off of at any given moment is what we have right now. Anything more is assumption, and any less is ignorance.
So, I can acknowledge that there's a
chance of an interstellar civilization having visited us in the past, while still pointing out that, based on what we know right now about how the universe works, it's very unlikely. Maybe the secret to interstellar travel is actually really simple. Maybe there are a
lot of alien species cruising around our universe - even in our immediate neighborhood - and they simply haven't contacted us for whatever reason. Maybe they
have contacted us, and buried somewhere in the bowels of conspiracy history is a startling truth.
But until the evidence for those things surfaces, if it ever does, we can't treat these as being on equal footing with what our instruments tell us - no one seems to be there, and if they're out there, we certainly haven't found them yet.
Note that I very much do believe we live in a universe teeming with intelligent life.