I doubt that
I doubt it/that
Something being discussed is or seems unlikely to happen. A: "Think Mom will let us have McDonald's for dinner tonight?" B: "I doubt it, since there's leftover meatloaf in the fridge." A: "I bet you got an A on the test." B: "I doubt that—there were a few questions that I totally guessed on."
I doubt that.
I do not believe that something is so. Bob: I'll be there exactly on time. Sue: I doubt that. John: Fred says he can't come to work because he's sick. Jane: I doubt that.