First evidence of dark matter started to appear in the 1970s. Vera Rubin was an astronomer who became famous for making this point and taking really exquisite observations that demonstrated that this is a real issue. In the 1980s people kind of took the idea and ran with it and said, well, if it's true that there's extra material out there, then it should have other consequences as well. This is where computer simulations come to the fore because it's very hard to figure that out just with pencil and paper.