German grammar question

Basically, after "Ich weiß nicht" I would expect the object of the not-knowing. "Weiß ich nicht" would be the reaction to an object just mentioned. Full version should mostly be "Das weiß ich nicht." In German a sentence can start with an object. Generally, objects would be found after the verb, but unlike English and French, they often enough also stand before the verb. German is very flexible. Basically, you would say the most important thing first... then the rest follows. And if the most important thing is the object? So what?

Hope this helps.