"Of course, Marxism is an example of what Carl Popper would have called a 'World Three' structure, in that it's got immense power as an idea, but you couldn't actually hold up anything in the world and say: 'this is Marxism'"

This quote by Alan Moore is referring to the idea of "World Three" as proposed by theorist Karl Popper. Popper argued that there are three worlds: the physical world, the psychological world, and the world of concepts. The real world is made up of concrete objects, the mental world is comprised of ideas and beliefs, and the world of ideas is made up of abstract ideas. Moore is stating that Marxism is an example of a World Three structure, because it is a concept with immense power, but it can not be physically seen or touched. Marxism is an ideology that has had a profound impact on the world, however it is not something that can be held in one's hand. Moore is highlighting the power of concepts and how they can shape the world, even if they can not be seen or touched.