The motivation behind calling it dark is mainly because it is really dark matter (in the sense that you cannot see it) that does not interact with electromagnet

The motivation behind calling it dark is mainly because it is really dark matter (in the sense that you cannot see it) that does not interact with electromagnetic force like ordinary matter which allows us to see it. And people usually refer to this work as dark sector which doesn't refer to the sector that we don't know (although we don't know much about in reality) but to the sector that is different from our ordinary matter.