Cheeky question:

If they've seen it, does that mean it's not dark any more?

NGL, it's like saying you see a shadow, though you're seeing the stark contrast from the (presumably) lit surroundings. Culturally acceptable, but is it scientifically accurate? 🤷🏼‍♂️

My understanding is that it's called dark because it doesn't interact with light or with matter except for via gravitational pull. So it is literally dark in that it can't be seen directly, only its effects observed. So in the event that we find some other way in which it visibly interacts with the world around it, "dark matter/energy" would probably be renamed to something better describing its behavior. I am not remotely qualified in this or any adjacent fields, so this is a layperson's understanding.