Flat-Earthers like to invoke the regress argument. They would keep asking “what is causing it?” on every statement in an attempt to turn the argument into an infinite regress. If they meet a question that gets unanswered, they would incorrectly declare the initial statement wrong.
In reality, we can know something exists even if we do not know what is causing it. Keep asking “what is causing it?” on pretty much anything will eventually give us an unanswerable question. But it does not mean we do not know it exists.
For example, if we can demonstrate the existence of gravity, they would ask what is causing gravity. Then, if we answer “the masses of objects,” they would keep pressing and ask why masses of objects create such a thing. If we answer they curve spacetime, they would continue and ask why they curve spacetime. And so on. Eventually, it will reach a question that we do not have the answer to. When it happens, they will declare victory and conclude that we do not know gravity exists.
In reality, we know gravity exists. We know how it affects things. We know how to measure it and predict the motion of objects. Just because we do not know everything about gravity, it does not mean we do not know anything about it.