OLogy Cards > 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.