Thinking about how little we know even about the amount of mass in our own solar system (Oort Cloud etc), could the discrepancies in mass simply be a ton of space junk (asteroids, planets etc) and even objects like cold neutron stars that are all very explainable objects in terms of their composition for the most part but are just really difficult to see? Now don't get me wrong, I would find it highly improbable that the types of particles that we commonly interact with and form the spectrum of chemical elements that we are accustomed to is the only type of particular matter in the universe, or even the only type that can form larger "chemical systems". I am skeptical of our ability to account for all the mass of the common types of matter, how this non-luminous is distributed in the galaxy (are there some kind of galaxy formation dynamics that cause more or less of the non-luminous mass- relative to star material- to be distributed at the edge etc?) I suppose I could keep going but I'm all questions and no answers. I'm just hoping someone can enlighten me to some of the stronger arguments for the existence of exotic dark matter vs. hard-to-see regular matter.