You could be right, Machiavelli, since both Hagen and the Don knew that Carlo was responsible for Sonny's death. Therefore, a "dark fact" possibility was that Connie would be widowed and her child(ren) made fatherless.
I think part of the confusion stems from Puzo's inaproppriate use of "fact." "Fact" implies something known. But in the same sentence, it's also something Hagen "dares not ask." We don't "ask" "facts." Therefore, I assumed it's really a "dark question": Will Michael take over the Family? Will Michael avenge Sonny in cold blood?