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.