IDK - "I Don't Know"
It is a common internet slang used as a short form while chatting. In a situation where you have no idea about someone's question, you can simple write "IDK". It conveys a message to the receiver, that you don't know anything about his question.
In a Conversation:
Person2: Hi, How are you
Person1: I went to see the park.
Person1: At what time does it open ?
Person2: IDK! i go around 4 PM.