Welcome back to "Doggerel," where I ramble on about words and phrases that are misused, abused, or just plain meaningless.
This line is often used to set up a textbook argument from ignorance: We don't know how X happened, therefore we know it's supernatural/aliens/whatever. As I've said to a few people who have used this technique, you can't put "I don't know" into an evidence locker. The lack of knowledge cannot be used as evidence for a positive hypothesis.
That's the short of the general form. Sometimes it gets downright ludicrous and appears to be stretched into the specific: "Some guy on the internet who calls himself Bronze Dog doesn't know, therefore we do know it's supernatural!" "Some specific expert doesn't know, therefore we know it's aliens!"
Science is a process of arriving at objective conclusions, and that involves making the experimenters, arguers, etcetera irrelevant to the interpretation of the data. It doesn't matter who is saying "I don't know." Just because something allegedly stumps the experts doesn't mean that you have free reign to declare you know the answer without evidence. If the people familiar with all the relevant theories and data don't know, you probably don't know, either, unless you have access to information the experts currently lack. If you do, I would suggest you present it instead of playing rhetorical trickery.
A major source of irritation this doggerel causes is that quite often, we do know. It's not very often that we stumble on true anomalies. Many arguments of this sort are centuries old and passed on by uncritical word of mouth.
The bigger irritation, I find, is that even if we don't know what caused X, it's a victory for no one. Just because science doesn't have all the answers doesn't mean that you can smugly make one up and defend it on an absence of evidence. Without evidence, you can't know, either.