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).
Most situations where people say, "I don't know" in response to a particular question is due simply to the lack of technology (which people are, indeed, building upon everyday which furthers our understanding of these unknowns) to gather sufficient evidence to reach a sound conclusion.
NOTE: This is not to be confused with I don't understand.