The proof is everything you know of that is physical is caused and not self explanatory. You have a fairy tale like this.
"Once upon a super-super long time ago, there was nothing. Not no-thing-nothing, but we'll say "nothing" as if it changes what that word has meant for millennia. Whatever it was, or wasn't, it was definitely not God. Certainly not God. This something-nothing huffed and puffed and blew the universe into existence! Out of not-nothing-but-not-God poofed an entire...something...from a not-nothing nothing. Or was that a something-not...Never mind.
Anyway…
So, the not-God-not-nothing-nothing exploded a universe that was super-perfect for life — like us — to exist. Even the teeniest, tiniest little change would mean no planets, no elements, no us! We're hoping...we mean, guessing...this was just good luck on the part of the nothing-something-blowing-in-ago-ness, but definitely not on purpose. Certainly not on purpose. Certainly not God. Life is possible only by luck!
And then, another amazing thing happened. This universe "evolved" which, like "nothing" is a word we'll use to mean pretty much anything whether it makes any sense or not. Even more amazingly, this change had nothing controlling or guiding it! By, "nothing," this time, we mean a complex series of consistent, independent, inexplicable and finely tuned rules. So, not nothing-nothing, but more of that something-nothing. You know, I mean, rules-from-nowhere-for-no-reason. Except the rule about things getting less ordered over time...forget that one for now. It all came together to make galaxies and stars and planets. We don't know why or how, but we know it was definitely not designed. Certainly not designed. Certainly not on purpose. Certainly not God. The not-guided-except-by-something-not-God-stuff self-made itself. Or was not-self-made...not into anything. Something? Never mind.
Anyway…
One little planet was not too far from a star, not too close to a star, but juuuust right so that complicated molecules could jiggle around and become even more complicated molecules and become reproducing organisms. We have no clue how this actually happened, but that doesn't stop us from being definitely, totally sure it involved absolutely no intelligence or design at all. We have fai— (ahem), confidence...confidence this moment or process or whatever was definitely not creation. Certainly not creation. Certainly not designed. Certainly not on purpose. Certainly not God.
Eventually, those organisms travelled through a magical journey of change and self-discovery to become...us! All without any meaning, purpose, reason, or guidance of any kind whatsoever! Which means you are...or, well...actually, it means you're just mindless matter reacting to physics, and everything you are or will be is going to come crashing down in obscurity and futility when the universe collapses. We know for sure, for sure that there is definitely no meaning to the universe. Certainly not meaning. Certainly not creation. Certainly not designed. Certainly not on purpose. Certainly not God. But we can make our own meaning, whatever that means.
And that's the moral of the story! This magical tale, about which we actually know almost nothing, and can prove exactly nothing, explains everything — everything — I SAID EVERYTHING! about life and our universe. "
Proof that is all there is to it.
I have a title for your story: the Strawman.
The simplifications and condescending language fail to address even the tip of the ice berg. And since you brought up God, which you were just complaining about earlier I might add, postulating a magical invisible infinite sky fairy ironically seems like far more of a fairy tale. God has always been and always is. At some point he decided to magically pop a universe into existence, and wave his wand to create life and us for absolutely no reason other than the fact that he was bored.
"Once upon a super-super long time ago, there was nothing."
False. My position has been that nothing has never existed and that is a philosophical imagining that is fundamentally flawed and contradictory.
"So, the not-God-not-nothing-nothing exploded a universe that was super-perfect for life "
Another oversimplification. The universe is far from perfect for life. In fact almost all of it isn't perfect for life. Constants and other universe could be more suitable for life. And a multiverse would have an infinite number of universes with different combinations. We exist in this universe because we could only exist in this particular universe. Many similar universes would exist with slightly different constants and initial conditions. The anthropic principle, nothing magical.
"One little planet was not too far from a star, not too close to a star, but juuuust right so that complicated molecules could jiggle around and become even more complicated molecules and become reproducing organisms."
With trillions and trillions of planets, at least a few of them were bound to be in the correct spot. It's unlikely for molecules to start self replicating, but given enough time and enough atoms in the universe its within the realm of possibility. It can happen by itself. Its within the laws of physics.
"Eventually, those organisms travelled through a magical journey of change and self-discovery to become...us! All without any meaning, purpose, reason, or guidance of any kind whatsoever!"
What is so great about purpose or guidance or meaning anyways? Overrated.
"We know for sure, for sure that there is definitely no meaning to the universe."
No one said this. We don't have evidence going either way, but there's no apparent meaning . The universe doesn't need meaning.
"Which means you are...or, well...actually, it means you're just mindless matter reacting to physics, and everything you are or will be is going to come crashing down in obscurity and futility when the universe collapses."
Which will happen with or without God.
"This magical tale, about which we actually know almost nothing, and can prove exactly nothing, explains everything — everything — I SAID EVERYTHING! about life and our universe."
And the same thing could be said about God.
"Proof that is all there is to it."
This barely addresses any of my points. This is quite possibly the poorest written and least logical piece of emotional pathos rhetoric ever produced.