so the other comment reply does nothing to explain, but there's too much mis- and disinformation that people consume and too much prideful ignorance to just let a comment sit like this as the top of the "hot" algorithm to not give people an entry point to enlighten themselves.

dark matter is not named for its undetectibility within the visible light spectrum. you are correct that we cannot detect much matter within the universe based on the visible light spectrum. dark matter is named based on that we cannot detect it at all. dark matter is more the answer to a question we don't yet know how to answer. there are a lot of behaviors out there in the cosmos that cannot be explained through the simple interactions between matter that we are able to observe, leading many scientists to theorize that much of the universe, around 70% of it, is made of a form of matter that interacts with the kind of matter we are by just kinda… pushing it away (not like anti matter which causes waveform collapses when interacting with the kind of matter we are).

a competing theory for why the cosmos acts the way it does is because space time itself is expanding as a result of our observable universe existing on the inside of a black hole. my estimation is probably that both explanations contain part of the truth and that the whole truth is that both dark matter exists and our universe is inside a blackhole. if you want a real mind trip, i suspect our universe is inside a blackhole inside our universe, and that every blackhole in the universe contains a universe. filled with blackholes with universes, including the very same universe that contains that blackhole