So we know everything, except for what we don't know. And we assume that what we don't know is complex, because otherwise we would surely know it. Yet history shows that's often far from the case. It's just that knowing what you don't know is rather difficult, even when the unknown is very simple, simply because you might not even stop to consider such a possibility. People thought the black plague was wrought and spread by evil spirits. They could have easily managed to prove it was being spread by rats, if they were so inclined, but who'd have thought to consider such things at the time?
For a contemporary example we are now discovering that the gut biome is seemingly associated, perhaps causally one way or the other, with all sort of other characteristics of the body and even the mind (as in the autism : gut biome link). We've had the technology to discover this for centuries, but who in the world would think to bother imagining a connection between your gut and anything besides ailments of the stomach?
Well, about that -
https://www.livescience.com/61444-black-death-cause-found-tr...