We can be pretty confident that life as we know it has evolved elsewhere, given the chemistry involved, but intelligent, (and more importantly) life that develops technological civilizations is much less certain.
Detecting intelligent non-technological life is likely impossible unless we actually visit such a planet. Consider a world where creatures like dolphins or whales are the dominant species. No fire (okay, sodium reactions, but how do you create and store pure sodium ions?), so no metallurgy.
As I said, I'd need to see some pretty convincing evidence (even with the vast expanses of both space and time in the universe pushing in that direction) to claim that intelligent, technological life exists elsewhere. That doesn't mean such civilizations don't exist, but given we don't really understand how intelligence (as a prerequisite for a technologically advanced civilization) comes about, except in broad evolutionary terms, making such a claim is premature IMHO.
But still, it's just unfathomable that in billions
of years across billions of trillions of star systems,
that only one developed intelligent life
It's also important IMO to understand that advanced intelligence is not an inevitable goal of evolution. Evolution only "cares" about survival. That can result in intelligent life, but it can just as often result in horseshoe crabs -- creatures of low intelligence that fill a niche perfectly and resiliently.
Intelligence is a really tough way to survive. You need a metabolically expensiv and fragile brain (or brains)
I'm a believer of the notion of "great filter(s)."
I think most civilizations on a path to interstellar travel have to pass through a nuclear fission phase (how do you build advanced stuff without discovering this?) and I think a great % of civilizations will end/cripple themselves during this phase via nuclear war. This will filter out a lot of civilizations.
The next challenging filter would be making serious efforts at exploring/colonizing/exploiting other planets and solar systems before exhausting their own home planet's resources or making it unlivable. The human race is definitely on pace to lose this race.
But still, it's just unfathomable that in billions of years across billions of trillions of star systems, that only one developed intelligent life. Even on earth, life was nearly destroyed 5 times, but still survived and evolved.
Of course, it's very likely that such life will be forever undetectable by us, and we'll be undetectable by them.