Every once in a while, a new scientific idea comes forth to challenge previous
modes of thinking. Copernicus' observation that the Earth is not the center of
the solar system (or universe) is an example of such a revolution in science
and theology.
Scientists recently underwent another such mind-boggling revolution when they realized that the majority of the universe is not made of the same type of matter as the Earth. From gravitational effects, we can infer the existence of this "dark matter," a type of matter that we cannot see. There is extensive circumstantial evidence that much of this is not made up of protons, neutrons, and electrons, as we are.
Not Dark Matter
What is dark matter? We don't know. Possibly dark matter is composed of neutrinos, or even more exotic forms of matter hypothesized by theorists.