1. Why Did Everyone Expect the Expansion of the Universe to Slow Down?
Imagine tossing a ball straight up. It leaves your hand fast, then slows as Earth's gravity pulls it back. Astronomers thought of the expanding universe the same way: after the big bang space kept expanding, and all the matter in it pulls back on itself. So the expansion should be slowing down. Astronomers debated how quickly it was slowing, and whether it might stop one day[1].
To test this, you would need to measure the speed of the expansion long ago and compare it with the speed today. And because light takes time to travel, looking at very distant galaxies means looking back in time. The trouble is that you need to know exactly how far away those galaxies are. That is hard, and it is where the story really begins.
2. How Can an Exploding Star Work as a Cosmic Measuring Stick?
Think of a row of identical streetlights. The farther away one is, the dimmer it looks. If you know that every lamp is the same, then how dim one looks tells you how far away it is. Astronomers call an object like this a standard candle.
The candle used here is a kind of exploding star called a Type Ia supernova. These explosions are so bright that they can be seen across billions of light-years, and they all have a similar true brightness. That means their apparent brightness gives a measure of distance[3]. By the 1990s, bigger telescopes in space and on the ground, and new digital cameras, made it possible to find and measure these supernovae in very distant galaxies[1].
The brightness of a supernova gives its distance, and the light of its galaxy tells you how fast that galaxy is moving away, because light from a receding galaxy is stretched to redder colors. Put the distance and the speed together for many supernovae, and you can see how the expansion has changed over time[3].
3. What Did the Two Teams Find When the Far-Away Supernovae Looked Too Dim?
Two groups of astronomers raced to do this, working separately. The Supernova Cosmology Project was led by Saul Perlmutter, and the High-z Supernova Search Team was led by Brian Schmidt, with Adam Riess playing a key role[1]. Both expected to see the expansion slowing down[3].
Instead, the far-away supernovae looked dimmer than they should have if the expansion were slowing. Dimmer means farther away than expected, and that means the universe had stretched more than the slow-down picture allows. The teams read this as evidence that the expansion is speeding up[2]. Both teams presented their findings in 1998[1].
The result was a shock, even to the people who found it. The Nobel committee wrote that the discovery "came as a complete surprise even to the Laureates themselves"[1]. Two independent teams reaching the same answer is what made scientists take it seriously. In 2011, Perlmutter, Schmidt and Riess shared the Nobel Prize in Physics "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae"[1].
One more detail may help. "Accelerating" here means that the speed at which far-apart galaxies separate keeps growing. It does not mean that you, or the Earth, are being flung around. Gravity still holds galaxies, the solar system and people together. The speeding up shows up only across the huge gaps between galaxy clusters.
4. What Does the Name "Dark Energy" Really Mean, and What Could It Be?
To explain the speeding-up, scientists use the name dark energy for whatever the unknown cause is. "Dark" here is a bit like it is in "dark matter": it means we cannot see it and do not know what it is. Despite the similar name, it is a different mystery, and our companion article on dark matter tells that story.
NASA lists a few main ideas for what dark energy might be[2]:
- A built-in energy of empty space (often called vacuum energy), a steady background energy that is part of space itself.
- Quintessence, a kind of energy field that fills space and could change strength over time.
- A flaw in our theory of gravity. Maybe there is no dark energy at all, and Einstein's general relativity needs fixing on the very largest scales.
None of these is confirmed. Several candidates have been proposed, and which one is right is still not known.
5. What Does Einstein's Old "Cosmological Constant" Have to Do With Dark Energy?
An older idea turns out to be connected. In 1917, Albert Einstein was using his new theory of gravity to model the whole universe. At that time most scientists believed the universe was static, neither growing nor shrinking, and his equations did not give a static universe, because gravity would make it collapse. So he added an extra term to the equations, now called the cosmological constant, which acts like a gentle push outward and balances gravity[4].
Later, once the universe was found to be expanding, there was no need to hold it static, and the term was set aside[4]. The physicist George Gamow later said Einstein called it his "biggest blunder". Historians have argued about whether Einstein really said that. In 2018 two researchers went through the evidence. They found that the remark fits Einstein's later cosmology and that at least three physicists reported it independently, and they concluded it is very likely he used the phrase on at least one occasion. It is a story passed on in other people's memories, not a line from his own writing[4].
After 1998, the cosmological constant came back into fashion. NASA says some scientists suggested that a small, non-zero value of it could explain the speeding up[2]. An energy that is the same everywhere and never changes would be the simplest dark energy. So a term that Einstein threw away for a wrong reason may turn out to describe something real, although that is not yet proven.
6. Is Dark Energy Constant, or Is It Changing? What Do the Newest Results Say?
If dark energy is a constant, it has had the same strength all through cosmic history. If it is something like quintessence, it might be getting stronger or weaker. To tell the two apart, astronomers need very precise maps of how matter spreads through the universe at different times.
One project doing this is DESI, the Dark Energy Spectroscopic Instrument, an instrument on a telescope in Arizona that records the light of very large numbers of galaxies and quasars. In March 2025, the DESI team released results from its first three years of data and said the evidence for a dark energy that changes over time had grown stronger than in the first-year results[5]. That created real excitement.
Then, in July 2026, DESI reported a new analysis based on a particular kind of data, the Lyman-alpha forest, which comes from hydrogen gas absorbing the light of distant quasars. The team says the measurement is more precise than before and its central value shifts toward the standard picture, which uses a constant dark energy. Two readings are possible: the hints of an evolving dark energy may fade away, or a more complex model may be needed to explain all the results at once[6]. So, as of October 2026, the question is still open.
More telescopes are coming. NASA describes the Roman Space Telescope as designed to study dark energy among other topics, and Europe's Euclid mission is making a 3D map of the universe to see how dark energy has pulled matter apart over time[2].
7. What Can You Look Up or Try Yourself?
You cannot build a supernova, but you can try the idea behind a standard candle. In a dark room, place a small battery-powered LED lamp or a phone flashlight at one end, and look at it from near and from far. It is the same lamp, but it looks dimmer from farther away. Ask yourself: if you did not know how far it was, how would you work it out? That is the astronomers' problem. (Do not use hot bulbs, and never point a strong light at anyone's eyes.)
To follow the story as it develops, look up the DESI project and the Roman and Euclid missions, and compare their announcements over the next few years. The Nobel Prize press release from 2011 is a short and readable account of the original discovery. A good question to keep in mind: why do two separate teams reaching the same answer count for so much?