Ideology. The Invisible Gorilla: And Other Ways Our Intuitions Deceive Us. Dance, mind & body. Dark Matter Is an Illusion, New Antigravity Theory Says. Dark matter is thought to be an invisible substance that makes up almost a quarter of the mass in the universe . The concept was first proposed in 1933 to explain why the outer galaxies in galaxy clusters orbit faster than they should, based on the galaxies' visible mass. (Related: "Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby? " ) At the observed speeds, the outer galaxies should be flung out into space, since the clusters don't appear to have enough mass to keep the galaxies at their edges gravitationally bound.