As commented many, many times, argument from ignorance is extremely prevalent in religion, ufology, new age, and all kinds of pseudoscience: The fact that science doesn't know something is taken as evidence of your favorite explanation.
Many pseudoscientists, however, go even farther. It's not enough for them to take things that we genuinely don't yet have an explanation for. No, they have to take things we know quite well how they work, and pretend that we don't. Just making the claim is enough to make many people believe that's so.
Example: "Science has no idea why water is the only substance that has a lower density when it freezes."
Present this claim to a fan audience, and they will swallow it without even a hint of skepticism. Just because a charismatic person is making such claims will automatically suspend any disbelief or skepticism in their audience.
In fact, there are two errors in that example. Firstly, water is not the only substance that has a lower density when it freezes. Secondly, we know quite well why water behaves in that manner. (Basically, the water molecules take more space when frozen because they rearrange themselves in a crystalline shape, and they rearrange themselves like that when frozen due to their electromagnetic properties.) Yet, how easy is it to sneak in not just one, but two errors like this, and have a gullible audience swallow it without question?
This is especially jarring nowadays when you can do fact-checking extremely easily by googling. We are today amazingly privileged in this manner compared to people of just 20 years ago who had no such resources at their disposal. Back then it could well take you a full day, or several days, to find the facts that you can find today in a couple of minutes from the comfort of your home. Yet people are still too lazy to do that.