I don't know.
我不知道。

I'm not sure.
我不确定。

Who knows?
谁知道啊？

Don't ask me!
别问我！

It's not certain.
还不确定。

Nothing is set yet.
事情都还没有定案。

It hasn't been decided yet.
还没有决定呢。

It's not clear.
事情还未明朗。

It's up in the air.
情况还混沌未明。

It's hard to say.
很难说。