You could also argue that God does not exist because there is no proof that he/it does exist but when you talk about faith then you do believe that God exists, this is not science. Likewise you could believe in the theory of creation not that of evolution likewise without supporting scientific proof.
However the probablity of life existing elsewhere in our universe is based on known science ie: how life formed on earth and mathematics, from these, conclusions are made that the probability of life existing elsewhere is statistically high given the right set of circumstances.
Argument from ignorance, also known as
argumentum ad ignorantiam or appeal to ignorance, is an
informal logical fallacy. It asserts that a proposition is necessarily true because it has not been
proven false (or vice versa). This represents a type of
false dichotomy in that it excludes a third option: there is insufficient investigation and the proposition has not yet been proven to be either true or false.
In debates, appeals to ignorance are sometimes used to shift the
burden of proof.
Carl Sagan famously criticized the practice by referring to it as "impatience with ambiguity", pointing out that "absence of evidence is not evidence of absence". This should not, however, be taken to mean that one can never possess evidence the something does not exist.