You are mistaking "theories" and "hypotheses". Theory in science is not some wild shot in the dark, imagined by some random guy in the eureka moment. And neither it is a something yet unproven. Theories in sciences are usually sufficiently proven and stand on the other previously proven theories. Like for example evolution of species is a theory, despite it having more than a century of research and hard proofs. So yeah, science is based on theories, but not on a collection of lucky guesses.

Now hypothesis is what you were probably mistaking a theory with. A hypothesis is something unproven and may or may not be a real thing.