If we knew nothing about dark matter, we wouldn't postulate dark matter. Astronomers define/describe dark matter as matter that seems to interact with normal m

If we knew nothing about dark matter, we wouldn't postulate dark matter. Astronomers define/describe dark matter as matter that seems to interact with normal matter gravitationally but in no other way. But we can "see" this gravitational interaction in a number of ways and so we know a reasonable amount about dark matter, especially, through the motion of normal matter we know approximately where it is and isn't.