You’re missing an important part: “and extra matter that doesn’t interact with the electromagnetic field would explain it.” It’s called dark because it doesn’t

You’re missing an important part: “and extra matter that doesn’t interact with the electromagnetic field would explain it.” It’s called dark because it doesn’t interact with the electromagnetic field (i.e. we cannot see it) and matter because it’s otherwise assumed to behave just like the matter we can see. It is a bit of a “We don’t like it, but it’s the best we can do for now”, but it is not like physicists invented something completely out of thin air to make the theory explain observations.