dark matter

Most mass in the universe is not normal matter we can see, like stars, planets, and puppies. In fact, 80% of the mass is called dark matter. Invisible, it doesn't give off light like a galaxy or absorb light like a black hole. Scientists know it exists because it has a gravitational pull, just like normal matter. Astronomer Fritz Zwicky proposed this "cold, dark material" in the 1930s. He realized that galaxies must be held together by dark matter's gravity.

Definition: an invisible substance that makes up 80% of the total mass in the universe
Detection: only by its gravitational influence on normal matter
Influence: its gravity holds together galaxies and galaxy clusters
Proposed: in 1930s by Fritz Zwicky
Confirmed: in 1978 by Vera Rubin. To explain the orbits of stars in spiral galaxies, she estimated that most matter in galaxies is invisible.

Image credits: main image, © AMNH.