Dark energy: The physics-breaking force that shapes our universe, explained

Physics is good at explaining many of our universe’s strange phenomena — from the way invisible beams of the light fan into rainbows to how rocket fuel can propel us into space.

Even so, there are still many bizarre events in our cosmos that evade even physics’ understanding and perhaps none so baffling as the existence of dark energy and dark matter.

These mysterious entities make up over 90 percent of “stuff” in the universe and may be the only thing keeping life as we know it from snuffing out in a cosmic “snap” or “crunch” of the universe itself.

Here’s what you need to know about dark energy — including how it’s different than dark matter and what it might be made of.

What is dark energy in simple terms?

Instead of describing an observed phenomenon — like inky, black energy tendrils radiating through space — the name “dark energy” instead refers to a gap in physicists' knowledge. If point A and point B are both observable events, then dark energy is the unknown link that connects them. In this case, the link that scientists believe explains why our universe’s expansion is speeding up instead of slowing down.

Likewise, instead of imagining this energy as something the color of coal, “dark” more so signifies that it’s invisible to both the human eye and human instruments. This invisibility seems to come from the fact that photons — or light particles — don’t interact with dark energy.

History of dark energy

We can trace the history of dark energy itself back to the first moments of the Big Bang, but human scientists didn’t begin to suspect its existence until billions of years later. Specifically, suspicion began in the early 1900s with Albert Einstein. At the time, scientists believed that the universe as we observed it was static — not growing or shrinking. However, this assessment didn’t jive with Einstein’s calculations. Instead, he believed that the universe was dynamic and could be either expanding or contracting.

This idea of an expanding or inflating universe was later proposed and picked up by the physics community in the 1980s. However, even then, the explanation wasn’t quite right. Scientists at the time believed that the universe had expanded after the Big Bang and had slowed its expansion since then. It wasn’t until 1998 that a supernova would flip this notion on its head.

Through studying a supernova event — the fiery final moments of a star’s life — researchers were able to peer into the past to observe the universe’s expansion rate. They found that the universe was expanding more slowly in the past, meaning that the expansion of our universe is accelerating instead of slowing down. This discovery would win the scientists a Nobel Prize in 2011.

Scientists dubbed the mysterious expansion force dark energy.

What is dark energy made of?

Because scientists still don’t really know what dark energy is (as in, what it does), they also aren’t very sure what it might be made of either. This in itself is pretty alarming as scientists have been able to measure the amount of dark energy (or, rather, the amount of energy that can’t be ascribed to “regular” energy), and it accounts for 68 percent of all “stuff” in the universe.

When you add dark matter to this mix, which accounts for about 27 percent of the universe’s contents, that means that only 5 percent of the universe is made of energy and matter that we can understand and observe. This makes dark energy and dark matter potent forces in our universe.

Even so, scientists haven’t let their uncertainty prevent them from postulating several different theories about what this invisible collection of dark energy is, including:

• A fifth fundamental force joining the likes of gravity, electromagnetism, and the strong and weak nuclear forces. Some scientists have dubbed this force or field “quintessence” in reference to Greek philosophers
• A fundamental property of space itself
• An error in Einstein’s theory of gravity

Until scientists can get closer to confirming one of these theories, all we can assume for sure about dark energy is that it’s responsible for our universe’s accelerating expansion — but even that may be proven false.

Dark energy vs. dark matter

Where dark energy is a repulsive force that stretches our universe, scientists have found that dark matter has the opposite effect. Dark matter acts as a parking brake to slow what could be a much more rapid expansion.

But that’s not the only thing dark matter invisibly impacts. Scientists also believe that dark matter is responsible for holding together many other parts of our universe. While we can’t see or interact with dark matter, scientists can measure the “imprint” it leaves, for example, a cluster of galaxies that behave much more massively than can be accounted for by “normal” matter.

Like dark energy, scientists aren’t sure what it’s made — but they do have a few ideas:

• An abundance of extremely light (hypothetical) particles called axions
• Fewer of an extremely heavy (theoretical) type of particle called WIMPS (Weakly Interacting Massive Particles)
• Primordial black holes from the beginning of time

How do scientists look for dark energy and dark matter?

While much of what we know about dark matter and dark energy are just theories right now, that doesn’t mean that scientists stay cooped up in their offices to study it.

Instead, dark matter and dark energy hunters turn their telescopes toward the sky to study the distortion of space or strange clumping of galaxies. Or, they try to detect interactions on Earth in highly specialized detectors, like XENON1T. In the past few years, scientists have reported that such detectors have caught glimpses of axions and dark energy alike, though the results aren’t conclusive.

It may still be many years — or even decades — until we truly understand these strange entities, but at least one thing’s certain: scientists won’t be stopping their search any time soon.