"The thing that makes cosmology different from stamp collecting is that we have a hope that there's a simple underlying theory that actually explains everything," Primack says. The current incarnation of that theory says that more than 90 percent of the universe is made up of mysterious "dark matter,"a fundamentally different kind of stuff than the protons, neutrons and electrons that make up Earth and everything on it. Dark matter is invisible (thus the name), and it interacts with ordinary matter only through the force of gravity. In a very loose way, the dark matter is like the water in the chunky noodle soup: we can't see it directly, but it must be there because the soup wouldn't look the same without it.