I think, philosophically, dark matter has been "seen" as much as anything. Like, radio waves are invisible, but you accept that we've "seen" them, right? So-called visible light isn't what activates your visual cortex, it's a cascade of rube goldberg machinery triggered by photons; the inside of one's head is dark. Nothing is seen directly, but all the things which we think we've seen have various intermediate steps before we can "see" them. We've detected gravity waves - is that "seeing", and do people who doubt dark matter exists doubt the gravity wave detectors work?