Historians are academics that source their findings and tell it as it was, right? Well, but to some extent you have to speculate, don't you? To what extent is that done and are there ever historical events that are debated in their very essence, or is it more a discussion of nuances amongst historians?
This varies extremely widely by field. Early precolonial African history: lots of speculation, because evidence is patchy. Twentieth-century American history: much less, because there is more evidence than we know what to do with. Speculation can be fine if there is truly no way to definitively answer a question, but professional norms demand that we be clear about what we have evidence to make a strong argument for, and what we are merely speculating about. And speculation should still be grounded in evidence, even if it's not quite enough evidence to draw a firm conclusion.