Astronomers classify all the stuff we can see in the universe as baryonic matter, made up of atoms and smaller subatomic particles. But starting in the 1970s, they began collecting evidence that there was more to the universe than meets the eye.
One of the biggest clues came when scientists tried to determine the masses of galaxies. They did this by measuring the acceleration of clouds orbiting on the outer edges of a galaxy, which enabled them to calculate the mass required to cause that acceleration. What they found was surprising: The mass behind the orbital acceleration of a galaxy’s clouds was five times larger than the mass of the baryonic matter spread across the galaxy. They concluded that there must be some invisible material surrounding a galaxy and holding it together. They called this material dark matter.
Twenty years later, scientists noticed that type Ia supernovae — dying stars that all have the same intrinsic brightness — were farther away from our galaxy than they should have been. To explain this observation, they suggested that the expansion of the universe is actually accelerating. This was perplexing, because the gravity inherent in dark matter should have been strong enough to prevent such an expansion. Was some other material, something with an antigravity effect, causing the rapid expansion of the universe? The astronomers believed so, and they called this material dark energy.
