Theory in science generally means something much more stringent than it does in vernacular. From Wikipedia:

A scientific theory is an explanation of an aspect of the natural world and universe that can be (or a fortiori, that has been) repeatedly tested and corroborated in accordance with the scientific method, using accepted protocols of observation, measurement, and evaluation of results. Where possible, theories are tested under controlled conditions in an experiment.[1][2] In circumstances not amenable to experimental testing, theories are evaluated through principles of abductive reasoning. 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 explains "why" or "how": a fact is a simple, basic observation, whereas a law is a statement (often a mathematical equation) about a relationship between facts and/or other laws.

So when something is being put forward as "A Scientific Theory" it is meant to be taken as the best possible explanation we can make of why the universe is the way it is, backed by exhaustive tests using the best methods currently available to us.

In science, when something is just a theory in the way you mean, it's called a hypothesis.