After darwin proposed his ideas about natural selection as a mechanism for evolution, it took time before evolution was generally considered a theory. why do we now refer to it as a theory?

It is a fact and not a theory

Explanation:

A scientific theory is some natural phenomenon that has been tried and tested several times and backed by logical and testable observations and hypotheses in a way that  in future anyone can test it. It is not a guess or hunch but a concept explained with logic and backed by solid unvarying proofs.

In case of biological evolution, observations have been made and tested several times so that no more further testing or trying is required. Hence, by this logic evolution can be considered as a fact but it can not be considered as a theory.

but there is no

i guess you forgot putting it.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.