Here's why:

* Theory vs. Hypothesis: A hypothesis is a proposed explanation for a phenomenon, while a theory is a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.

* Experimental Evidence: Theories are built upon a strong foundation of experimental evidence. This means they have been repeatedly tested and verified through rigorous scientific methods.

* Wide Acceptance: Theories are not simply accepted on a whim. They gain acceptance within the scientific community through a process of peer review and critical analysis.

Examples:

* The theory of evolution: Explains the diversity of life on Earth through natural selection and other mechanisms, supported by vast amounts of evidence from fossils, genetics, and observation.

* The theory of gravity: Explains the attraction between objects with mass, supported by countless experiments and observations, including the movement of planets and the falling of objects.

It's important to note that even though a scientific theory is widely accepted, it's not considered "proven" in the absolute sense. Science is always evolving, and new evidence could lead to modifications or refinements of existing theories.