Answer: A phenomenon
Explanation: A phenomenon is a fact which can be observed,theory tries to show the relationship between facts and events. A phenomenon is different from a noumenon which comprise of facts or events which can not be observed. The term came into recent use by Immanuel Kant,who tried to compare between noumenon and phenomenon.
Theories are of great importance in the world of science as it has been useful in giving explanations to many events, happenings and observations,through the concepts, assumptions or proposition and definition it contains.