The question "what is truth?" is as old as time, and so important that we've created a system to analyze it and attempt to come to a consensus[0]. If an idea is untestable, it is logical and good for your sanity to assume that it is false until proven otherwise.
We assume that certain aspects of reality are real because they are testable and repeatable. We say gravity exists because every time we drop something, it falls down towards Earth, and if we apply other assumptions while looking out in the sky, we see other objects falling towards each other. The assumption is logical and testable.
It is impossible to test whether we just materialized, therefore it's safe to assume it did not happen unless repeatable testing indicates otherwise.