A fallacy wherein it is asserted that no possibility, no matter how absurd, can be ruled out unless explicitly disproven. Sometimes, the justification for this is the presence of a deity or other supernatural being; aliens from outer space possessing advanced technology, etc.
Often an attempt at ShiftingTheBurdenOfProof to the point of absurdity, and/or an attempt to expand the universe of discourse to something so large that nothing conclusive can be said at all. (This is often done by proponents of a discredited idea; so that in the wider UniverseOfDiscourse the evidence against their idea no longer has sufficient standing to be an effective argument).
Understood by most as a metaphor. Meaning not "Anything at all", but that things not imagined as possible, things having never been done, things which had formerly been considered as impossible, are possible. Sometimes only due to concerted, dedicated, systematic hard work and application of science, art, and business acumen. The proof is in the pudding, not in the theory, argument or logic of opposers.
Good example of your meaning: People used to believe that man would never exceed the SpeedOfSound. Today, it is thought that one can never exceed the SpeedOfLight. Just a matter of time! (no pun intended!) -- BrucePennington
Anything is possible in the next half hour! -- from a television series
See also: SoftwareGivesUsGodLikePowers