Right, but it gets worse. Philosophically (my undergrad major) it is impossible to prove a negative. Ask them to lay out the case that there are no unicorns, Yeti, or other mythical creatures. I cannot prove that I have never not seen a space alien.
You told it: it's a TROLL! Then believers ( either they are trolls or not), perfectly know that when something doesn't exist, so it can't be proven ( and will never be).