Proving a Negative
In most cases, you cannot “prove” a negative, because an example of the thing you’re trying to disprove may exist, you just haven’t seen it yet. However, you can prove that something is impossible or prove that there is no evidence for a thing.
Examples
Absolute atheists may claim “It is proven that God doesn’t exist,” however this really hasn’t been proven. More accurately, they should say, “I have seen no evidence that indicates God exists,” or “The claims attributed to God are impossibilities, therefore it is highly doubtful that God exists.”
Technically, we don’t know that unicorns don’t exist. Just because we’ve never seen one, doesn’t mean there’s not one grazing in a field somewhere.