"Datta" is the daily form from the formal-form of "deshita"
Formal "are/is" form in Japanese:
Desu (current) -> De wa arimasen (In no-current and the same time no-after) -> Deshita (after)
Daily "is/are" form in Japanese:
Da (-,,-) -> De wa nai/Ja nai (-,,-) -> Datta (-,,-)
Wakatta?...