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.

Image credits: main image, © AMNH.