모르겠다 has the emphasis of "wouldn't know". also ~겠다 has the meaning of "I guess/suppose" etc. While 모르다 (몰라(요)), just means "I don't know". Furthermore, I think 모르겠다 express a little bit more uncertainty whether you know it or not, while with 모르다 you are certain you don't know.

모르겠어요 and 몰라요 essentially mean the same thing. They both mean "I don't know" except 모르겠어요 is a bit more polite. They also both indicate present tense.