How do we know the universe as we know it does not exist as a single atom in a much bigger universe?

I think we know enough about subatomic particles and Planck's constant to say that our atoms don't contain their own universes.

Could I keep making up ridiculous scenarios and not ever be proved "wrong"?

Yes, but this line of thinking is futile. If I told you I had tentacles coming out of my ears you'd be hard pressed to prove me wrong, but that doesn't make it true. This is called the burden of proof - you make the claim, you substantiate it. It's not the job of other people to prove you're just talking out of your ass.

science is WEIRD

Well, none of those things are science, really. At best they're untested hypotheses. They're fun to speculate about, but none of them are grounded in studies of our reality.

First and foremost, science is a process of putting ideas to the test. You get an idea and ask, "If this is true, how could we tell? What clues do we look for in the world to evaluate our idea? What experiments can we design to generate some relevant data?"

Then you run your experiments or perform your observations, and you compare your results to the idea. If they match up: congratulations! You may be on to something. Now run the experiment a hundred more times to reduce the possibility of error, publish your results to a scientific journal, and wait while the whole scientific community dissects and repeats your experiment looking for any trace of bias. If your results don't match the idea, either toss it out or change it to match the observed facts.

But with an idea like "What if our universe is one atom in a bigger universe"... there's just no way to ever test it. The physics of our universe place a hard limit on how far we can see in any direction. Same with Last-Thursdayism: there's no experiment you could run, no study you could perform to prove or disprove it. It just hovers in the limbo-realm of "Technically not disproven, even though nothing we can see indicates it to be true."

This gives rise to a favorite philosophical razor of mine, known as Newton's Flaming Laser Sword, which goes something like "If something cannot be settled by experiment or observation then it is not worthy of debate."

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We don't.