I actually consider that a fair challenge - it's just not proof. "I suspect X, does anyone have data to invalidate that?" is a great question. You just have to

I actually consider that a fair challenge - it's just not proof. "I suspect X, does anyone have data to invalidate that?" is a great question. You just have to avoid argument from ignorance, and follow up with "If not, can we get data to actually support this theory?"