The short answer is: we don't know. But we do know that it exists, that it is causing the universe to expand at an accelerating rate, and that about 68.3 to 70 percent of the universe is dark energy.

Dark energy was not discovered until the late 1990s. But its origins in scientific research date back to 1912, when American astronomer Henrietta Swan Leavitt made an important discovery using Cepheid variables, a class of stars whose brightness fluctuates with a regularity that depends on the brightness of the star.

All Cepheid stars with a certain period (the period of a Cepheid is the time it takes to go from bright to dim to bright again) have the same absolute magnitude, or luminosity—that is, the amount of light they emit. Leavitt measured these stars and showed that there is a relationship between their regular period of brightness and their luminosity. Leavitt's findings allowed astronomers to use a star's period and luminosity to measure the distance between us and Cepheid stars in distant galaxies (and in our own Milky Way).

Around this same point in history, astronomer Vesto Slipher observed spiral galaxies using his telescope’s spectrograph, which is a device that splits light into its component colors, much like a prism splits light into a rainbow. Slipher used the spectrograph, a relatively new invention at the time, to look at different wavelengths of light coming from galaxies in different spectral lines. With his observations, Slipher was the first astronomer to observe the speed at which galaxies were moving away from us—a phenomenon called redshift—in distant galaxies. These observations would prove to be critical to many future scientific advances, including the discovery of dark energy.

Redshift is a term used when astronomical objects are moving away from us and the light coming from those objects is stretched out. Light behaves like a wave, and red light has the longest wavelength. Thus, light coming from objects moving away from us has a longer wavelength, extending into the “red end” of the electromagnetic spectrum.

Eventually, the discovery of galactic redshift, knowledge of the period-luminosity relationship of Cepheid variables, and the new ability to measure the distance to stars or galaxies led astronomers to realize that galaxies were moving away from us over time, showing how the universe was expanding. In the years that followed, scientists around the world began to piece together the story of an expanding universe.

In 1922, Russian scientist and mathematician Alexander Friedmann published a paper detailing several possibilities for explaining the history of the universe. The paper, which was based on Albert Einstein's theory of general relativity published in 1917, included the possibility that the universe was expanding.

In 1927, Belgian astronomer Georges Lemaître, who is said to have been unaware of Friedmann's work, published a paper that also took into account Einstein's theory of general relativity. And, although Einstein claimed in his theory that the universe was static, Lemaître showed how the equations in Einstein's theory actually support the idea that the universe is not static, but is in fact expanding.

Astronomer Edwin Hubble confirmed in 1929 that the universe was expanding, using data from his colleague, astronomer Milton Humason. Humason measured the redshift of spiral galaxies. Hubble and Humason then studied the Cepheid stars in those galaxies, using the stars to determine how far away their galaxies (or nebulae, as they called them) were. They compared the distances of these galaxies to their redshift and observed that the farther away an object is, the greater its redshift and the faster it is moving away from us. This pair of astronomers discovered that objects like galaxies are moving away from Earth faster the farther away they are, at upwards of hundreds of thousands of kilometres per second – this is an observation now known as Hubble’s Law or the Hubble-Lemaître law. The universe, they confirmed, really is expanding.

Scientists previously thought that over time the expansion of the universe would likely slow down due to gravity, an expectation supported by Einstein's theory of general relativity. But everything changed in 1998, when two different teams of astronomers observing distant supernovae noticed that (at a certain redshift) the stellar explosions were fainter than expected. These groups were led by astronomers Adam Riess, Saul Perlmutter, and Brian Schmidt. The trio won the Nobel Prize in Physics in 2011 for this work.

While faint supernovae may not seem like a major find, these astronomers were looking for Type 1a supernovae, which are known to have a certain level of luminosity. So they knew there had to be another factor making these objects appear dimmer. Scientists can determine the distance (and speed) of an object using its brightness, and dimmer objects are typically farther away (although surrounding dust and other factors can make an object appear dimmer).

This led scientists to conclude that these supernovae were much farther away than they expected by observing their redshift.

Using the brightness of the objects, the researchers determined the distance of these supernovae. And using the spectrum, they were able to determine the redshift of these objects, and therefore how fast they were moving away from us. They found that the supernovae were not as close as expected, meaning that they had moved away from us faster than expected. These observations led the scientists to conclude that ultimately the universe itself must be expanding faster over time.

While other possible explanations for these observations have been explored, astronomers studying supernovae or other even more distant cosmic phenomena in recent years have continued to gather evidence supporting the idea that the universe is expanding faster over time, a phenomenon now called cosmic acceleration.

But as scientists gathered evidence of cosmic acceleration, they also wondered: Why? What could be driving the universe to expand faster over time?

That's where dark energy comes in.

Right now, dark energy is just the name astronomers have given to the mysterious “something” that is causing the universe to expand at an accelerating rate.

Some have described dark energy as having the effect of a negative pressure pushing space outward. However, we don't know if dark energy has the effect of any kind of force at all. There are many ideas about what dark energy might be. Here are four main explanations for dark energy (note that it's possible that dark energy is something completely different).

Vacuum energy:

Some scientists think dark energy is a fundamental, ever-present background energy in space known as vacuum energy, which may be equal to the cosmological constant, a mathematical term used in the equations of Einstein’s theory of general relativity. Originally, the constant existed to counteract gravity, resulting in a static universe. But when Hubble confirmed that the universe was actually expanding, Einstein eliminated the constant, calling it “my biggest mistake,” according to physicist George Gamow.

But when it was later discovered that the expansion of the universe was actually accelerating, some scientists suggested that there might in fact be a non-zero value for the previously discredited cosmological constant. They suggested that this additional force would be necessary to speed up the expansion of the universe. With this, it was postulated that this mysterious component could be attributed to something called “vacuum energy,” which is a theoretical energy that is deep within the universe and permeates all of space.

Space is never exactly empty. According to quantum field theory, there are virtual particles, or pairs of particles and antiparticles. These virtual particles are thought to cancel each other out almost as soon as they emerge into the universe, and this act of popping in and out of existence could be made possible by the “vacuum energy” that fills the cosmos and pushes space outward.

While this theory has been a popular topic of discussion, scientists investigating this option have calculated how much vacuum energy should theoretically exist in space. They showed that there should be so much vacuum energy that, in the beginning, the universe would have expanded outward so rapidly and so strongly that no stars or galaxies could have formed… and there should be absolutely nothing. This means that the amount of vacuum energy in the cosmos must be much smaller than it is in these predictions. However, this discrepancy has not yet been resolved and has even earned the nickname “the cosmological constant problem.”

The quintessence:

Some scientists think dark energy might be a type of fluid or energy field that fills space, behaves oppositely to normal matter, and can vary in its amount and distribution over both time and space. This hypothetical version of dark energy has been dubbed quintessence, after the theoretical fifth element discussed by ancient Greek philosophers.

Some scientists have even suggested that quintessence could be a combination of dark energy and dark matter, although the two are currently considered to be completely separate. While both are big mysteries to scientists, dark matter is thought to make up about 85% of all matter in the universe.

Wrinkles in space:

Some scientists think dark energy might be a kind of defect in the fabric of the universe itself; defects like cosmic strings, which are hypothetical one-dimensional “wrinkles,” are thought to have formed in the early universe.

A flaw in general relativity:

Some scientists think that dark energy is not a physical thing that we can discover. Instead, they think there might be a problem with general relativity and Einstein's theory of gravity and how it works on the scale of the observable universe. Within this explanation, scientists think it's possible to modify our understanding of gravity in a way that explains observations of the universe that have been made without the need for dark energy. In fact, Einstein proposed such an idea in 1919, called unimodular gravity, which is a modified version of general relativity that scientists today think would not require dark energy to make sense of the universe.

Dark energy is one of the great mysteries of the universe. For decades, scientists have developed theories about our expanding universe. Now, for the first time, we have tools powerful enough to test these theories and really investigate the big question: “What is dark energy?”

NASA is playing a key role in the ESA (European Space Agency) Euclid mission , launching in 2023, which will create a 3D map of the universe to observe how matter has been torn apart by dark energy over time. This map will include observations of billions of galaxies up to 10 billion light-years from Earth.

NASA’s Nancy Grace Roman Space Telescope, scheduled to launch in May 2027, is designed to investigate dark energy, among many other scientific topics, and will also create a 3D map of dark matter . Roman’s resolution will be as sharp as NASA’s Hubble Space Telescope but with a 100 times larger field of view, allowing it to capture larger images of the universe. This will allow scientists to map how matter is structured and spread throughout the universe and explore how dark energy behaves and has changed over time. Roman will also conduct an additional survey to detect Type Ia supernovae.

In addition to NASA’s missions and efforts, the Vera C. Rubin Observatory, supported by a large collaboration including the U.S. National Science Foundation and currently under construction in Chile, is also poised to support our growing understanding of dark energy. This ground-based observatory is expected to be operational by 2025.

The combined efforts of Euclid, Roman and Rubin will usher in a new “golden age” of cosmology, in which scientists will gather more detailed information than ever before about the great mysteries of dark energy.

In addition, NASA's James Webb Space Telescope (launched in 2021), the world's most powerful and largest space telescope, aims to make contributions to various research areas and will contribute to studies on dark energy.

NASA’s Spectrophotometer for Universe History, Epoch of Reionization, and Ice Explorer (SPHEREx) mission, scheduled to launch no later than April 2025, aims to probe the origins of the universe. Scientists hope that data collected by SPHEREx, which will survey the entire sky in near-infrared light, including more than 450 million galaxies, can contribute to a better understanding of dark energy.

NASA also supports a citizen science project called Dark Energy Explorers, which allows anyone in the world, even those without a scientific background, to help in the search for answers about dark energy.

*A short note*

Finally, to clarify, dark energy is not the same as dark matter. Their main similarity is that we don't know what they are yet!

By Chelsea Gohd
NASA Jet Propulsion Laboratory