This unseen "stuff" is postulated to exist because of the way its gravitational influence is seen to affect the rotation of galaxies. But its substance is unknown because no other type of interaction has yet been observed.
There are certain theories of dark matter where it barely interacts with the regular world at all, such that we could have a dark matter planet exactly co-incident with Earth and never know. Maybe dark matter people are walking all around us and through us, maybe my house is in the Times Square of a great dark matter city, maybe a few meters away from me a dark matter blogger is writing on his dark matter computer about how weird it would be if there was a light matter person he couldn’t see right next to him.