7. Why do astronomers believe there must be dark matter that is not in the form of atoms with protons and neutrons?
Galaxies could not have formed as early as they did without dark matter gravitationally attracting ordinary matter and inducing galactic formation. The existence of dark matter is also necessary to explain the long-term stability of both spiral galaxies and galactic clusters. In both cases, we see material in their outer regions moving around their centers too fast for the gravity we deduce from ordinary matter to hold. There must be some other form of material there with gravity. Yet searches for electromagnetic radiation from this additional matter have been fruitless, leading scientists to believe that this "dark matter" does not consist of ordinary particles, such as protons and neutrons.
