The Big Bang & the universe
Dark Energy Explained Simply: the engine of the universe
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For a long time researchers thought the expansion of the universe must be slowing down. Gravity should put on the brakes. But the opposite is true: the cosmos is getting ever faster.
What dark energy is
Dark energy is an unknown form of energy that fills all of space. It acts like a pressure that drives the cosmos apart.
Unlike matter, it does not attract but in a sense repels. It is the reason the expansion of the universe is accelerating.
The discovery in 1998
Until the late 1990s, the open question was how fast the expansion was braking. Two teams measured distant supernovae as distance markers for this.
The result was a shock. The expansion is not braking, it is accelerating. For this discovery the 2011 Nobel Prize in Physics was awarded.
How it works
The more space stretches, the more dark energy seemingly appears. Because its density stays the same, while matter is spread ever thinner.
So dark energy gains the upper hand over time. A phase first dominated by matter became one ruled by acceleration.
What it could be made of
The simplest explanation is the cosmological constant. It describes a constant energy of empty space that Einstein once wrote into his own equations.
Other models suspect a changing field. Which idea is right is open. This is exactly where one of the deepest puzzles of modern physics lies.
Dark energy and dark matter
The names sound similar, yet the two are opponents. Dark matter attracts and builds up structures like galaxies.
Dark energy, by contrast, drives everything apart. Together with ordinary matter they make up the full budget of the cosmos, visible also in redshift.
What it means for the future
If the acceleration continues, distant galaxies drift away from us ever faster. In the very far future they would no longer be visible.
The exact fate depends on whether dark energy is really constant. Even small deviations would have enormous consequences.
Why it is the greatest puzzle
Dark energy is the largest component of the universe and at the same time the least understood. We measure its effect precisely but do not know its nature.
Understanding it could fundamentally change physics. That is why it stands at the center of cosmology, as the Big Bang section shows.
Frequently asked questions
Is dark energy the same as dark matter?
No. Dark matter attracts through gravity and holds galaxies together. Dark energy acts in the opposite way and drives the expansion of the universe.
What is dark energy made of?
That is unknown. The simplest idea is a constant energy of empty space, the cosmological constant. Whether that is right only the future can show.
How much of the universe is dark energy?
About 68 percent. That makes it the largest share of all, ahead of dark matter at around 27 percent and ordinary matter at only about 5 percent.
How do we know dark energy exists?
In the late 1990s, measurements of distant supernovae showed that the universe expands ever faster. Something must drive this acceleration, and that is called dark energy.
Will dark energy tear the universe apart?
Possibly, but uncertain. If it stays constant, everything expands forever. If it grows, a Big Rip threatens in the far future. Which scenario holds is open.
Did anyone win a Nobel Prize for dark energy?
Yes. In 2011 the Nobel Prize in Physics went to the researchers who detected the accelerating expansion in supernovae. That was the decisive clue for dark energy.
Sources and further reading
- Dark Energy — NASA
- Accelerating Universe — ESA
Update note (as of: 06/05/2026)
First publication of the dark energy article.
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