I am not a scientist, but I do like to watch them on youtube: We're pretty sure something we don't know about has a ton of mass. Otherwise a bunch of other stuff doesn't really make sense, for example the universe should be way way bigger than it is now without something unseen keeping it condensed. So the existence of "dark matter" is about as confirmed as it can be without observing it directly. What we don't know is what dark matter is, how to observe it, or how it works. But math, as we currently understand it, implies its existence pretty heavily.

Which does in fact mean that our math or understanding could just be wrong. Technically you're right, we DON'T know if there really is dark matter. But if we assume our current knowledge is correct we can make a pretty reasonable guess about it.