He's a pretty straight shooter, but then I think I'm the only who calls McCoy Bones to his face, so...
[ He ducks when she reaches out on instinct, except it went through them? He glances at himself and then her, but doesn't ask the immediate question. Staying alive is more important, and he can grill her about being incorporeal once they were out of the sector. ]
How else? Killing us off. There was a big war, a lot of weapons, a lot of blood. Well before my time, but it was a turning point in human history.