A recent letter stated, "It would be interesting to read your explanation of the origin of the whole universe via the doctrine of evolution."
Often, the big bang theory and evolution are confused and conflated. In the same way the big bang theory does not demonstrate the diversity of species, evolution does not explain how the universe came into existence.
Evolution starts explaining the diversity of species only after the emergence of complex life on Earth. What happened before that is the realm of other scientific fields.
In common usage, a theory is a feeling or feasibility. This is not the way the term is used in science.
In science, a theory is a well-established explanation for scientific data — such as the theory of gravity or the Pythagorean theorem.
Evolution has passed every test used to validate how such a huge diversity of species exist on earth. As scientific instruments and methods have improved over the decades, evolution has continued to be confirmed.