But it is visible, it’s visible in terms of gravitational effects. We can “see” the effects of dark matter. That is evidence specifically for dark matter, i.e. matter that is very hard or impossible to detect via the electromagnetic spectrum but is observable through gravity.
Dark matter is the explanation, the question is more what form does it take.
It just takes a bit of acknowledgement that actually the EM spectrum is not the only way to view the universe. In fact it’s just one of four (maybe five) fundamental forces. We’re just used to that being the default for seeing because it’s how we physically see. It’s an anthropocentric bias to say something doesn’t exist because we can’t view it via EM radiation despite the fact gravity is clearly showing it to us.
You could use your logic to argue against the existence of black holes. We don’t see them by definition but they are most certainly there.