In 1929, American astronomer Edwin Hubble studied a number of exploding stars, or supernova, and determined that the universe was expanding. The notion that distant galaxies were moving away from ours was a radical idea.

It seemed obvious to astronomers that gravity—the mutual attraction between all matter—would affect the expansion process. But how? Would the pull of gravity completely halt the expansion of the universe? Could the universe stop expanding and then reverse itself back toward us? Or would the universe eventually escape the gravitational effect and continue to expand? The universe may be expanding, reasoned the scientific community, but its expansion was surely slowed by the forceful effects of gravity.

Fast forward nearly 70 years to a time when two teams of astrophysicists—one led by Saul Perlmutter at the Lawrence Berkeley National Laboratory and the other by Brian Schmidt at Australian National University—began studying supernovas to calculate the assumed deceleration of expansion. To their astonishment, they discovered that supernovas as far as 7 billion light-years away were not brighter than expected but rather dimmer, meaning they were more distant than the teams had calculated them to be. The universe isn’t slowing down, they concluded. It’s speeding up.

The discovery turned the scientific world on its head: If gravity isn’t the most dominant force in the universe, what is? In 1998, American theoretical cosmologist Michael S. Turner dubbed the mysterious new something “dark energy.” Yet even with a name, we know little about dark energy.

Theorists have come up with several explanations for dark energy. The leading theory claims that dark energy is a property of space. Albert Einstein claimed it is possible for more space to come into existence and that “empty space” can have its own energy. “As more space comes into existence,” reports NASA, “more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster.”

NASA reports that scientists have been able to theorize how much dark energy there is out there because we know how it affects the expansion of the universe. Roughly 69 percent of the universe is dark energy. Dark matter accounts for about 27 percent, leaving the rest—all normal matter, everywhere—adding up to less than 5 percent of the universe.

Another explanation posits that dark energy is a new type of energy field or energy fluid that fills space but affects the expansion of the universe differently than matter and normal energy. Scientists have labeled this energy “quintessence,” but we still don’t know what it interacts with or why it even exists.