There's a very commonly used argumentative fallacy used by many people, the so-called argument from ignorance (argumentum ad ignorantiam). The name of this argument is a bit unfortunately named because it's really confusing. People very often misunderstand what it actually means, and is misleads people easily.
Many people think that it means something like "making claims about a subject that one understands poorly". In other words, pretty much "you don't know what you are talking about".
For example, many people could call something like this an "argument from ignorance":
"The Occupy Wall Street movement just wants anarchy, thus they shouldn't be taken seriously."
While that's certainly a completely stupid argument, it's not an argument from ignorance, because that's not what the term means.
What it means is to argue for a position using an unknown, something unexplained. In other words, the very fact that something is unexplained is used as an argument pro some claim or position. Perhaps a better, less ambiguous name for the argument would be "argument from an unknown".
For example, a very, very common argument for the existence of a god is: "Science can't explain where the universe came from." In other words, something that is unexplained is taken as evidence for the existence of something else. This, of course, is just fallacious argumentation. You can't just jump from an unknown to some conclusion. That's just faulty logic.
Another common example is using something unexplained, or hard to explain, in photographs and videos to argue for the existence of ghosts, UFOs, the paranormal, or a myriad of other things.
Not being able to explain something is not evidence of something else, nor does it even give it any kind of credibility. That's just not how it works. If it's unexplained, then it's unexplained, period. No conclusions should be drawn from that fact.