If any human being is intelligent, then human life is intelligent.
If human life is intelligent, then there is intelligence in the Universe.
If there is intelligence in the Universe in one minute region, then the odds that there is intelligence in other minute regions are far more likely than they are unlikely.
If the odds of intelligence are more likely than unlikely, then the odds of that intelligence being similar to human intelligence is also more likely than unlikely.
There are between 200 billion to 2 trillion galaxies in the observable Universe.
Each galaxy may contain upwards of 10,000,000,000,000 solar masses— that is, stars or the equivalent. While it is unknown whether all those stars are orbited by planetary masses, the likelihood is far greater that they are than that they are not.
In our own solar system there are well over twenty (possibly even over 100) planetary bodies with sufficient mass to render them spherical and thus to have significant gravity conducive to life (not that gravity is strictly necessary for life), and a significant number possessing liquid water in some form (though in most cases buried under massive crusts of water ice), or other liquids functioning more or less as water does on Earth (Titan's liquid methane seas). Life, in some form, is possible on many of these bodies, though as of yet our technology and exploration efforts are insufficient to discover such life— even potentially intelligent life— though certainly human-like intelligence is less likely outside of what we now call the "Goldilocks" or "habitable" zone of our own star (roughly the orbit of Venus to Mars, give or take a few million miles). All stars have some similar zone, though certain stars known for high levels of radiation or periodic excessive radiation emissions might well not have such zones, at least for Earth-like life. However, that number is not all stars, nor even most stars.
Thus, it is more likely than not that any given Sun-like star will have planets orbiting it. It is more likely than not that such planets will possess water (hydrogen being the most abundant element in the Universe, and water being a common form of it, either as liquid or ice or vapor). It is more likely than not that such a planet will be within said star's "Goldilocks" zone for Earth-like life.
Thus, while intelligent life may not yet have been detected, and in fact may not be detectable to us using our current capabilities, the likelihood that it does exist is far greater than that it does not. Indeed, I would go so far as to assert, on the observational evidence provide above, that it is a certainty.
Any assertion that it does not exist is the opposite of science— it's just being contrary. Asking for proof, however, is certainly science— but assuming that truth will *never* be provided is not science, or that the failure to provide it definitively today (or ever) rules out the possibility, even the likelihood that intelligent life similar to our own intelligence is "out there."
The above is not consensus. It is actually the foundation of science— observation leading to hypothesis. What has not (yet) been achieved is experimental or observational proof.
Now, if one is arguing for life capable of interstellar travel via constructed devices, that's something different. We've only just now reached the possibility of that— though our "interstellar travel" would be at sublight speeds and take centuries, if not millennia, to reach even our closest neighboring stars. But we could do it. Already our emissaries have left the Solar System for parts unknown— Voyagers I and 2 are on their way to the distant stars. And if we can do that, then others could to. Can we do more? Can others do more? The jury on that is still out, and falls under speculation— "informed speculation" at best, but still speculation.
