What Is Dark Energy?

Dark energy is the name scientists give to whatever is making the expansion of the universe speed up. It is a label for an effect, not an explanation. The speed-up itself is well established. Two teams discovered it in 1998, and three of their leaders shared the 2011 Nobel Prize in Physics. By current estimates, dark energy makes up roughly 68 to 70 percent of everything in the universe (NASA).

What dark energy actually is remains one of the deepest mysteries in physics. The simplest idea is that empty space itself carries a small, steady energy. But recent surveys have raised a startling possibility: dark energy might be slowly weakening over time. That hint is intriguing. It is not settled.

How did astronomers find out the universe is speeding up?

Astronomers have known for about a century that the universe is expanding. Distant galaxies are moving away from us, and the farther they are, the faster they recede. Most scientists expected gravity to be slowing that expansion, the way a ball thrown upward slows as it rises.

To check, two teams, one led by Saul Perlmutter and the other by Brian Schmidt with Adam Riess, studied a special kind of exploding star called a type Ia supernova. These explosions reach a predictable peak brightness, and a single one can shine as brightly as a whole galaxy. That makes them useful as “standard candles.” If you know how bright a light bulb really is, how dim it looks tells you how far away it is.

In 1998, the teams reported that some 50 distant supernovae looked fainter than expected. They were farther away than a slowing universe would allow. The expansion is speeding up. It was a complete surprise, even to the researchers. The official Nobel Prize explainer compares it to throwing a ball in the air and watching it race away faster and faster. By its account, the expansion slowed for billions of years, then began speeding up about five to six billion years ago.

What does “dark energy” actually mean?

It is a placeholder. NASA describes dark energy as the name astronomers gave to the mysterious “something” driving the speed-up. Whatever it is, it seems to belong to space itself. Early on, it hardly mattered. But as the universe grew, matter spread thinner and its gravity weakened. Dark energy did not thin out the same way, so its push eventually took over.

Dark energy is not the same as dark matter. Their names sound alike, but their effects are opposite. Dark matter’s gravity pulls things together. Dark energy seems to push the universe apart. In both cases, “dark” simply means we cannot see it and do not yet understand it. (For the other mystery, see What Is Dark Matter?)

Is it Einstein’s cosmological constant?

It might be. When Einstein applied general relativity to the whole universe, he added an extra term, the cosmological constant, to hold the universe still. Once astronomers found that the universe was expanding, he dropped it. The Nobel explainer says he came to see adding it as a big mistake. After 1998, it made a comeback. A small, constant energy of empty space would produce exactly the kind of steady push that speeds up expansion. The Nobel explainer’s verdict: Einstein added the constant for the wrong reasons, but it turned out to be brilliant.

The idea has a famous problem. Quantum physics says empty space is never truly empty. It seethes with fleeting activity that should carry energy. But the simplest estimate of that energy misses the measured value by a factor of about 1 followed by 120 zeros. NASA calls this “the cosmological constant problem,” and it remains unsolved. Other proposals include quintessence, an energy field that could change over time, and the possibility that Einstein’s theory of gravity needs adjusting on the largest scales.

Is dark energy changing over time?

This is where the news is. The Dark Energy Spectroscopic Instrument, or DESI, works on a telescope at Kitt Peak in Arizona. It is building the largest 3D map of the universe. DESI measures baryon acoustic oscillations, a faint pattern in how matter is spread out, left by sound waves in the early universe. That pattern works like a ruler printed across the cosmos. By measuring how big the ruler looks at different distances, scientists can track dark energy over the last 11 billion years.

In March 2025, DESI reported results from its first three years, covering nearly 15 million galaxies and quasars. On its own, DESI’s data fit the standard model, in which dark energy is a cosmological constant. But combined with other measurements, the data leaned toward dark energy that weakens over time (Berkeley Lab). The strength of that preference ranged from 2.8 to 4.2 sigma, a measure of how unlikely a result is to be a statistical fluke. Physicists usually want 5 sigma before claiming a discovery. As the DESI team itself noted, many 3-sigma results have faded with more data.

Since then, the picture has become more mixed. A recalibrated analysis of Dark Energy Survey supernovae found that the preference dropped from 4.2 to 3.2 sigma, which its authors call only a weak preference (Popovic and colleagues). In July 2026, a new DESI measurement using hydrogen gas in the distant universe agreed with the standard model. DESI’s own summary says this could mean the hint will fade, or that an even more complex model is needed (DESI). The honest bottom line: nobody knows yet.

What do we still not know?

Almost everything about its nature. We don’t know whether dark energy is truly constant, why its strength is so tiny compared with theory, or whether it is a new ingredient at all rather than a sign that our theory of gravity is incomplete. Philosophers of science also point out that many dark energy models are tailored to fit the observations rather than predicted in advance (Stanford Encyclopedia of Philosophy). That difference is explored in Fitting data versus making a prediction.

The answer also shapes the far future. If the speed-up continues, the Nobel explainer says the universe will probably end “in ice,” spread through ever colder and emptier space. If dark energy changes over time, that fate could be different. For now, the long-range forecast is uncertain.

What would change the answer?

• More DESI data. In April 2026, DESI finished its planned five-year map, with more than 47 million galaxies and quasars, and it keeps observing. Its first dark energy results from the full five years are expected in 2027.
• Rubin Observatory. In June 2026, the Vera C. Rubin Observatory in Chile began a ten-year survey that will observe the entire southern sky every few nights.
• Euclid and Roman. ESA’s Euclid telescope, launched in 2023, is building a 3D map of billions of galaxies. NASA’s new Roman Space Telescope will run its own search for type Ia supernovae.
• Agreement across methods. If supernovae, galaxy maps, and the cosmic microwave background all show dark energy changing, with high confidence, the cosmological constant would be in real trouble. If the hint fades, the simplest picture stands.

Dark energy is a reminder that the biggest part of the universe is the part we understand least. It is also a live story: the next few years of data could confirm a picture nearly 30 years old, or overturn it. For the wider backdrop, What Does the Big Bang Actually Explain? is a good next step. And keep asking the simplest question of all: what is empty space, really?