Speculation is fine. It's impossible to go through life without guessing. But what you shouldn't do is make the leap from speculation to conclusion without the evidence to support it.
A scientist will often say "I think" or "This could be." But he/she cannot say "I know" until it is proven.
There are vast area of reality that are unexplored. There are even things we simply can't know. Anyone interested might profit from a review of "Heisenberg's uncertainty principle", "Schroedinger's Cat" and the limits of knowledge.
But, as you point out, just because there are wide swaths of area we know little about and some things we cannot know this doesn't justify or excuse a headlong leap into supernaturalism, wild speculation and superstition.