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 decided¹ 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。