Argumentum ad ignorantiam
The argumentum ad ignorantiam, also known as the argument from ignorance or argument from (personal) incredulity, is a logical fallacy wherein the speaker claims that because a subject is not well understood, either by the speaker or in general, it cannot be true.
The argumentum ad ignorantiam is commonly used as a proof of the existence of God. The argument goes like this: "How could this big, complicated world have come into existence without an intelligence behind it?"
This argument is also used to dismiss the possibility of biological evolution, in this fashion: "Evolution says that people just came into existence by chance. This is clearly preposterous; I cannot see how this is possible, so evolution must be false."
Irreducible complexity is also based on the argument from ignorance.
Often a subject such as evolution is not understood by the speaker, but well understood by many others, such as scienctists. Of course, evolution is not a theory of chance, and has well established mechanisms.
Even when a subject is not well understood (i.e., the origin of the universe) that is not sufficient grounds for assuming an unproven answer like God. Since the "explanation" of God is more complex than the entities that are explained by God, introducing God without evidence is simply begging the question.