"Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."
― Charles Darwin
Mike Pence does not understand that evolution occurred is a fact, and that collections of facts help comprise this robust scientific theory. In everyday use, theory may mean a guess or a hunch, something that may require proof. In science, a theory is not a guess, not a hunch ― it's a well-substantiated, well-supported, well-documented explanation for our observations. As we continue to make discoveries and obtain new evidence, the theory is refined and made stronger. In some cases this may require that parts of the theory are replaced or modified as we improve our understanding, but it does not change the fact that evolution occurred, and is occurring. Because he doesn't understand science, Pence speaks as if this is a weakness, but it's the great strength of the scientific method.