They calculated the size and effect of SOMETHING in the universe. They describe this something as dark matter. But that doesn't prove that dark matter exists or that they were actually observing it, or that dark matter is what they think it is.
For example I can observe a tree's leaves shaking and theorize that the wind is blowing, but in reality there could be plenty of reasons why that tree is shaking.
They named it dark matter in order to hypothesize that something existed, rather than describing it as dark matter beforehand, and observing it in the link I posted was what proved that this placeholder concept of dark matter did indeed refer to something real and measurable.