A scientific theory is an explanation of an aspect of the natural world and universe that can be repeatedly tested and corroborated in accordance with the scientific method, using accepted protocols of observation, measurement, and evaluation of results. Established scientific theories have withstood rigorous scrutiny and embody scientific knowledge. A scientific theory differs from a scientific fact or scientific law in that a theory seeks to explain "why" or "how", whereas a fact is a simple, basic observation and a law is an empirical description of a relationship between facts and/or other laws.

https://en.wikipedia.org/wiki/Scientific_theory

You're making a common mistake in thinking a scientific theory is the same as a layman's theory.