Guessing: Imagining theories to explain facts one does not know an explanation for.
This is how all human knowledge starts: By some guess that subsequently is confirmed by induction, perhaps with some corrections, and remains not refuted, and in the end may be used to produce technology.
Here is a relevant quotation, from Chamfort: Ce que j'ai appris je ne le sais plus; le peu que je sais encore, je l'ai deviné (= What I learned I no longer know; the little I still know, I guessed). Most supposed human knowledge consists of guesses that are more or less well-confirmed.
See also: Abduction.