all hypotheses are "unsubstantiated guesses" if you wish to look at it that way. I don't put them in the true guess category because there is some narrowing down of possibilities based on what is known already about anything and everything. It is more of an "educated guess" than just a random thought. There is reason behind the hypothesis even if the substantiation is absent. Naturally, when the hypothesis is investigated, that adds to the constraints, so even if the hypothesis proves to be incorrect (and that happens a lot), knowledge has increased as a consequence. The next hypothesis will be even more "educated" or reasoned. Eventually the hypothesis works out, sometimes, albeit rarely, on the very first "guess".