What is Dark Energy and How Do We Know It Exists?

95 percent of the universe is concealed from us, but we are coming closer to understanding those substances.

What is stuff made of? Any grade school student can tell you: protons, neutrons, and electrons make up the matter that comprises everything around you. Even the air is full of all different kinds of molecules, not to mention airborne microbes that are still too tiny for us to see or feel.

But the relatively solid world we live in is an anomaly: matter, as most of us know it, only makes up about 5% of the universe. But the rest of that vast expanse is far from empty. It’s full of dark energy and dark matter, theoretical substances that scientists can’t detect but which we know must exist based on otherwise unexplainable phenomena—phenomena like the expansion of the universe.

Scientists first discovered that the universe is expanding in 1998, using the Hubble Space Telescope. Not only that, its expansion is accelerating every year, which the four fundamental physical forces known at the time couldn’t explain . If only gravity was at play, the expansion initiated by the Big Bang would eventually start to slow down and perhaps even begin to contract. Physicists decided that a fifth force, or “quintessence,” had to be the culprit, and thus dark energy made its theoretical debut.

Dark energy employs a kind of reverse gravity that repels instead of attracting, pushing everything apart. It makes up nearly 70 percent of all energy in the universe, which explains what seems to be the vast emptiness between galaxies. But physicists aren’t sure of the exact nature of dark energy, largely because they have not yet been able to detect it.

A theory proposed in 2004 speculated that dark energy particles are actually hiding from detection, like chameleons blending into the background. Instead of changing colors, chameleon particles can change their mass to match surrounding matter. That explains why they’ve lurked out of sight all this time: in a high-density setting like Earth, chameleon particles exert little force, but in the vacuum of space they have free reign.

But recent insights in theoretical physics may finally reveal the true character of dark energy. A team of scientists at the University of California, Berkeley, decided to try measuring the minute forces acting on atoms in free fall. They placed an aluminum sphere and a dust-mote of cesium atoms into a vacuum chamber that mimics deep space conditions, called an atom interferometer.

If chameleon particles had formed a field within the interferometer, they would have caused the cesium atoms to accelerate towards the sphere. Instead, the atoms behaved as they normally do under the force of gravity. While the results did not unmask dark energy once and for all, they do establish that chameleon particles operate on a much smaller scale than gravity does. This sets the stage for future experiments that may finally confirm or refute the chameleon theory.

And once we pin down dark energy, we can get a better grasp on the fate of the universe. If dark energy continues to exert its repulsion between galaxies, the universe will continue to expand. Recent measurements of local electromagnetic radiation bolster this prophecy: the universe’s energy output is only half what it was 2 billion years ago, and it continues to wane.

Stars, the sources of light and warmth that fill our night sky, will drift further apart, eventually resulting in the “Big Freeze” scenario: the universe evolves into a large, dark, and cold void. At least we will better understand the forces at work behind dark energy long before it fulfills its destiny.

If you liked this article, learn about dark matter in Dark Materials Part II.