That's because a theory is an explanation of the way the world works.

Well...let's be more specific. That is: let's use more specific wording. *** Biological evolution has been observed by scientists, both in laboratory experiments and in nature. That biological evolution occurs is a fact. *** However, the scientific theory of evolution proposes far more than the mere sometime occurrence of biological evolution. For just one example: it also proposes that ALL life on Earth - that ALL species of life - are the result of evolution. (In other words: that at one time there was ONE species, and that ONE species evolved into the millions or however many different species that we recognize today). That has NOT been observed in the lab or nature, because to observe it ***you would have to actively observe all life from the first species to the present time***. *** SO: the theory of evolution remains a scientific theory because several of the tenets of the theory of evolution have never been proved - or "scientifically established", if you prefer that terminology. Only SOME of the theory of evolution - including, importantly, the occurrence of biological evolution itself - have been observed by scientists.

In science, a theory is an explanation for a phenomenon that can be tested and verified by the scientific method. It includes multiple sources of evidence that build up into a framework of understanding that can change over time to account for new discoveries and new evidence. Originally written about by Charles Darwin's Grandfather Erasmus Darwin, (33rd degree freemason and member of the Luna Society), he wrote Zoonomia where he claimed that all warm-blooded animals were descended from one common ancestor. He later wrote that “all vegetables and animals now existing were originally derived from the smallest microscopic ones, formed by spontaneous vitality in primeval oceans.”