Here's why:
* Nobody is a negative subject.
* Tag questions need to create the opposite polarity (positive or negative) of the main sentence.
* Since the main sentence is negative, the tag question must be positive.
* The auxiliary verb used in the tag question should match the tense of the main sentence. In this case, the present tense "knows" is paired with the present tense "do".
Therefore, the full sentence with the tag question is: Nobody knows, do they?