I don't know
"I don't know" is a statement that means the we do not have the answer to a particular question.
Many people are uncomfortable with the notion of human ignorance. In the face of our ignorance, many people feel the need to incorrectly insert a deity into the picture to explain away the unknown(merely an Argument from ignorance.
In most situations, "I don't know," is simply telling the truth of the matter - such as, we don't know definitively how the universe came to be or what the meaning of life is (if there is one).