Do I have any unquestionable proof? The answer is a clear no. But do I possess the ability to accept that, in all probability, there is 'life' out there, then the answer would be yes.
It cannot be believed that the Earth is unique and it is the only planet that is capable of sustaining life, in any form. If our planet was able to journey the path of evolution and be able to reach its current state of development, then there is no plausible scientific explanation to dismiss the fact that there are other forms of life present within the known universe. The only problem currently being experienced is with the inability to gather actual proof.
Currently mankind can only observe and speculate as to what is known as fact. Future generations and technological advancements will undoubtedly provide the opportunity to explore further into what we currently know and understand , and lead us to make new discoveries.
Man is basing its current investigations into what it has classed as the ability to sustain life. The continuous quest to seek out new world's that theoretically provide the capabilities. But this is assuming that all organisms require the same properties as we deem necessary to 'live'.
Man is currently on a continuous path of enlightenment and each new discovery and progression concludes a further stage of evolution. If we, as combined species on Earth, can survive then there is no reason to suggest that alternative forms cannot, and are not, currently existing within the known and unknown universe.
As to what level of evolution we determine this confirmation as, then only time will tell. At present science still speculates as to whether it will merely be a primitive form of bacteria, or an equivalent to us.
It cannot be foretold as to when and how contact we be assumed, and to what effect this will have upon how we see and view ourselves, but in all probability the time will come when we realize and have to accept, due to factual evidence, that the universe contains more life than planet Earth. Whether this will occur within my lifetime, I cannot say.
There is alternative life forms out there, we just do not currently possess the capabilities to confirm and to prove this as fact. In time mankind will be able to travel the galaxies in its search for new planets, and return with actual evidence that we are not alone. The next stage will be if we can truly accept this, and to what impact will this bring upon the human race.