It’s true that often you need to do research in order to find factual information, and if that’s the case, you need to indicate where you got those facts. But you also have many facts at your fingertips because of your reading and general knowledge. Depending upon your topic and your background, you might know historical facts or facts from current events on a local or global scale. If you’re writing an argument that’s a movie review, for instance, your knowledge of that movie can serve as evidence to make your point.