Dark Matter is the unseen matter that forms “halos” of galaxies and clusters of galaxies. There is much more of DM than luminous matter. Fritz Zwicky suspected it in 1933, strong evidence in 1970 from work by Vera Rubin and others. DM clumps exactly like ordinary matter. We do not know exactly what particle(s) it’s made up of, but searches are in progress. Most of DM is non-baryonic – not protons, neutrons etc.