I was actually writing an answer to your question "Does lack of evidence prove that something does not exist?" when you chose a best answer before I could post my response. Since I was writing in part a direct response to the answer that you chose as best, I thought I'd try to get it to you since you might find it interesting. But first I'll answer this question!
I'm an atheist, and I really enjoy science fiction and fantasy in general, but there's a lot of work in those genres that I'm less fond of. Some sci-fi and fantasy are written with a strong message of something like, "Don't trust in human reason, because the universe is filled with forces more powerful and more wise than we." I'm not as into that type of sci-fi/fantasy, because it doesn't resonate with me. Certainly the universe is a mysterious place, but I firmly believe that human reason is the best tool we have for navigating it, and for coming to fully appreciate how awe-inspiringly beautiful it is. I think the best sci-fi and fantasy have themes like that.
Now, your previous question, just in case you find it interesting:
The absence of evidence for something does not prove that the thing does not exist. However, there can be positive evidence for nonexistence. For instance, if I say that there is a mighty dragon living in my back yard that flies around my town every Thursday and burns down houses, you could collect evidence that this is untrue. You could visit my town and see that there are no burned buildings, and you could talk to my neighbors and discover that they know nothing about such events, and you could visit my back yard and see that there is no dragon. This would all be positive evidence that the dragon does not exist.
However, I might then claim that the dragon is invisible, and that the people in my town are very quick at rebuilding after the dragon burns things down, and furthermore that my neighbors are engaged in a conspiracy to lie to outsiders to keep them from knowing about the dragon. But now I am describing a different dragon than before, one that I have constructed very carefully to prevent positive evidence of its nonexistence from being found. While you would have difficulty finding evidence of this dragon's nonexistence, it would be unreasonable for you to conclude that I am probably telling the truth, or even that I have a good chance of telling the truth. The claim I am making for this dragon's existence is so far outside the bounds of normal human experiences that you should be skeptical. Certainly you cannot prove the dragon does not exist, but based on the lack of positive evidence in favor of the dragon along with the unfalsifiable nature of my claim, you would be justified in deciding that you do not believe in my dragon. A much simpler explanation is that I had fabricated the whole story, or perhaps that I am delusional and in need of psychiatric care.
Now, as for this "theory of observational existence" that Natasha has described. It is not nearly as profound as it might first appear. If we are to believe it, we must, as Natasha says, "redefine existence." So the definition for existence becomes that a thing meets condition A: being present in the universe, and condition B: being known about by a conscious agent. Sure, we can redefine this term if we wish. But as we'll see, this is nothing more than some word trickery.
When observational existence states that a thing may exist to one person but not to another, this is because of a difference in whether condition B (knowledge) is satisfied for those two people, but condition A (being) has not changed. So if my cat (which I know about and you don't) exists for me and not for you, that is a statement about you and me, not a statement about my cat. Likewise, when observational existence states that things may come into existence in the past when we discover them, this is again a statement about ourselves, not a statement about time. To claim otherwise is disingenuous. It would be like me saying that the best answer to a Y!A question is subjective depending on what each person thinks, and therefore because I always think my answer is best, I have a 100% Best Answer rating on Y!A. That's a statement just about my own opinion, not about objective truth.
There is another sneaky way people subscribing to this view might try to get around the problem, which is to reject condition A entirely. In other words, a thing exists ONLY if a conscious agent knows about it. But once again, this is nothing more than a definition. In fact, we have defined "existence" to be a synonym of "knowledge," or perhaps more precisely, "belief" (since the only difference between knowing something and believing something is whether that thing is independently true). The fact that we have defined the word existence in this way doesn't change the fact that condition A may or may not be satisfied by any given thing in the universe.
The thing is