The theory basically goes that any civilisation which could evolve to a 'post-human' stage would almost certainly learn to run simulations on the scale of a universe. And that given the size of reality - billions of worlds, around billions of suns - it is fairly likely that if this is possible, it has already happened.
Photo taken by Arjan Benning: This person's website has such an extensive portfolio I couldn't find information to anitate this! But I love the idea! It would be great to try in class!