Dark energy facts for kids

Kids Encyclopedia Facts

Dark energy is the name given to the force that is believed to be making the universe larger. Imagine the entire universe is like a giant balloon, and galaxies are like tiny dots painted on its surface. If you start blowing up the balloon, the dots move farther and farther apart. Scientists have observed that the universe is doing something similar – it's expanding! But here's the really surprising part: the expansion isn't slowing down, as you might expect from gravity pulling everything together. Instead, it's speeding up! Scientists call the mysterious force that seems to be pushing the universe to expand faster and faster "dark energy."

Distant galaxies appear to be moving away from us at high speed: the idea is that the universe is getting bigger and has been since the Big Bang. Measurements are now accurate enough to allow astronomers to tell that these galaxies seem to be accelerating away from us. The universe is expanding at an increasing rate.

What is dark energy?
How was dark energy discovered?
What could dark energy be?
Dark energy vs. Dark matter
Why is it Important to Study Dark Energy?
How are scientists studying dark energy today?
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What is dark energy?

Dark energy is a type of energy that scientists believe is spread throughout all of space. It's called "dark" because we can't see it, touch it, or measure it directly with our tools. We only know it's there because of the effect it has on the universe – it seems to be causing the universe's expansion to accelerate. Think of it like an invisible force, almost like "anti-gravity," that pushes things away from each other on the largest scales.

Scientists estimate that dark energy makes up a huge part of the universe – about 68% of everything in it! That's much more than all the stars, planets, galaxies, and everything else we can see, which only make up about 5% of the universe. Another mysterious substance called dark matter makes up about 27%. So, most of the universe is made of stuff we can't see or fully understand yet!

How was dark energy discovered?

For a long time, scientists knew the universe was expanding ever since Edwin Hubble observed in the 1920s that distant galaxies were moving away from us. They thought that gravity, the force that pulls things together, would eventually slow down this expansion, maybe even causing the universe to start shrinking someday.

But in the late 1990s, something unexpected happened. Two teams of astronomers were studying very distant exploding stars called supernovae. These particular types of supernovae, called Type Ia supernovae, are like cosmic lighthouses because they always have about the same true brightness. By measuring how bright they appear from Earth, scientists can figure out how far away they are.

The teams, one led by Saul Perlmutter and the other by Brian Schmidt and Adam Riess, measured the distances to these supernovae and how fast they were moving away from us. They expected to see the expansion slowing down over time. Instead, their observations showed that the supernovae that exploded when the universe was younger were farther away than they should have been if the expansion was slowing. This meant the universe's expansion must be speeding up! This surprising discovery in 1998 suggested that there was something unknown counteracting gravity and pushing the universe to expand faster. This is what scientists started calling dark energy. Adam Riess, Saul Perlmutter, and Brian Schmidt were awarded the Nobel Prize in Physics in 2011 for this groundbreaking discovery.

What could dark energy be?

Scientists don't know exactly what dark energy is, and it's one of the biggest mysteries in science today. They have several ideas, but none of them fully explain everything.

One idea is that dark energy is a property of space itself. Albert Einstein had an idea like this a long time ago, which he called the "cosmological constant." He thought it was a force that balanced gravity to keep the universe from collapsing. When the universe was found to be expanding, he thought this idea was a mistake. But now, the idea of a constant energy in space is back as a possible explanation for dark energy. If space itself has energy, then as the universe expands and creates more space, more of this energy would appear, causing the expansion to accelerate.

Another idea is that dark energy is a new kind of energy field or particle that we haven't discovered yet. Scientists have given names to some of these ideas, like "quintessence," but so far, there's no direct evidence for them.

Some recent research, like the data released in March 2025 from the Dark Energy Spectroscopic Instrument (DESI), has even hinted that dark energy might not be constant and could be changing over time. If this is true, it would be a very big deal and could mean scientists need to update their understanding of how the universe works.

Dark energy vs. Dark matter

It's easy to get dark energy and dark matter confused because they both have the word "dark" in them, and we can't see either of them directly. However, they are very different!

Dark Matter: This is invisible stuff that has gravity and pulls things together. Scientists know it's there because of how its gravity affects the way galaxies spin and how light bends around large groups of galaxies. Dark matter acts like an invisible glue that helps hold galaxies and clusters of galaxies together. It makes up about 27% of the universe.
Dark Energy: This is the mysterious force that seems to be pushing space apart, causing the universe's expansion to speed up. It acts like "anti-gravity" on the largest scales. Dark energy makes up about 68% of the universe.

So, while dark matter pulls things together, dark energy pushes them apart.

Why is it Important to Study Dark Energy?

Understanding dark energy is important because it affects the past, present, and future of the universe.

By studying how dark energy has affected the universe's expansion over billions of years, scientists can learn more about how the universe grew and changed after the Big Bang.
Dark energy is the biggest part of the universe today, so understanding it is key to understanding what the universe is made of and how it works.
The amount and behavior of dark energy will determine the ultimate fate of the universe. If dark energy continues to push space apart faster and faster, galaxies will keep moving away from each other, and eventually, the universe might become a very cold and empty place.

How are scientists studying dark energy today?

Since we can't see dark energy directly, scientists study its effects on the universe using powerful telescopes and experiments. As of 2025, several exciting projects are helping us learn more:

The Dark Energy Spectroscopic Instrument (DESI): This project is creating the largest 3D map of the universe by measuring the positions and distances of millions of galaxies and quasars (very bright, distant objects powered by black holes). By looking at how these objects are spread out, scientists can see how dark energy has influenced the universe's structure over the last 11 billion years. DESI released new data in March 2025 that provides the most precise measurements yet and hints that dark energy might be changing over time. The DESI experiment is planned to continue collecting data for five years.
The Euclid Mission: This mission from the European Space Agency, with help from NASA, launched in 2023. It is also creating a 3D map of the universe to study how dark energy has pulled matter apart over time.
NASA's Nancy Grace Roman Space Telescope: This new space telescope, planned to launch by May 2027, will have a huge field of view, allowing it to capture large parts of the sky at once. It will help scientists map how matter is spread out and study how dark energy behaves and might have changed over time.
NASA's SPHEREx Mission: Scheduled to launch no later than April 2025, this mission will survey the entire sky in infrared light, looking at over 450 million galaxies. The data from SPHEREx could also help scientists understand dark energy better.
The Vera C. Rubin Observatory: This ground-based telescope in Chile is expected to be fully operational in 2025. It will take pictures of the entire southern sky every few nights, creating a huge amount of data that will be used to study dark energy and dark matter.

These projects, along with others, are working together to gather more information than ever before about dark energy. Scientists are also exploring different theories and using powerful computers to simulate the universe and see if their ideas about dark energy match what they observe.

Images for kids

Diagram representing the accelerated expansion of the universe due to dark energy.
A Type Ia supernova (bright spot on the bottom-left) near a galaxy
Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data.