and there I gave a great quote from Prof. Strassler that dark matter is: “something that gravitates and clumps, but doesn’t shine, either in visible light or in any other form of electromagnetic waves”
so you can observe its presence through gravity, i.e. it clumps together but you cannot see it using our usual arsenal of electromagnetic waves (light, infra red, radio, gamma ray)…hence it is Dark, but since it interacts via gravity, it is also matter.
This stuff makes up 75% of matter so it is important. Its nature is not yet known and all sorts of wierd ideas have been put forward.