Its source is known. Unfortunately, it requires a different way of looking at everything. (It's all waves, even if it looks like a particle most of the time). Wrapping this up as simple pop science, that can be digested by most laymen, is difficult.

What we don't actually know is why everything is made of waves. We know the rules it follows, but not the underlying cause. Figuring that would would likely require an understanding of quantum relativity, something we only have a very weak handle on.

It's just proof that light behaves as a wave, because it generates an interference pattern like the first picture. The second picture is how it would theoretically behave if it was (only) a particle, which it isn't. The proof that light acts like a particle comes from a second experiment proposed by Einstein dealing with the photoelectric effect.

This article is meant for kids but it explains things pretty plainly. The duality is unexplained, but the experiment is well understood.

I vaguely remember an explanation that whatever device/mechanism is actually used to “observe” the experiment was affecting the behavior of the light. Boiling it down to “observation changed the outcome” makes you picture something that changes depending on whether you look at it with your eyes, but there’s a lot more moving parts to the whole thing.

Depends on what you mean by unknown. The meme (and a lot of common understanding) doesn't know what it means to be observed. There is a leading theory, the Copenhagen interpretation. The biggest theory in opposition would be multi-world.

Idk, but you might find the answer to that question from one of these

https://www.youtube.com/playlist?list=PLuqavL7RN4jjdioivWm-Jwa6IoGG2_8Nr