What is dark energy in simple terms?

Dark energy is the name for whatever is making the universe expand faster and faster. It seems to be spread evenly through space and makes up about 68% of the universe's energy content, but its true nature is unknown.

How do we know the universe's expansion is accelerating?

Distant Type Ia supernovae appear fainter, and therefore farther away, than they would be if expansion were slowing. This result has since been backed up by the cosmic microwave background and maps of how galaxies cluster across cosmic time.

Is dark energy changing over time?

Possibly. Results from DESI in 2024 and 2025 hint that dark energy may have weakened slightly over billions of years, but the evidence is not yet strong enough to be certain. Further data from DESI, Euclid and other surveys should settle the question.

What is the Hubble tension?

It is a disagreement between two ways of measuring the universe's current expansion rate. Predictions from the early universe give about 67–68 km/s per megaparsec, while direct local measurements give about 73, and the gap has not gone away as measurements improved.

Will dark energy rip the universe apart?

Not if it behaves like a cosmological constant, which current data broadly favour. In that case galaxies, stars and planets stay bound together while distant galaxies drift out of sight. A 'Big Rip' would require dark energy to grow stronger over time.

The mysterious force speeding up cosmic expansion

Dark Energy

Accelerating expansion · Everywhere

Since the 1920s astronomers have known that the universe is expanding: distant galaxies are moving away from us, and the farther they are, the faster they recede. For most of the twentieth century everyone assumed gravity must be slowing that expansion down. In 1998 two teams studying exploding stars found the opposite. The expansion has been speeding up for the past several billion years.

Whatever is causing this acceleration is called dark energy. It appears to make up about 68% of the universe's total energy content, more than dark matter and ordinary matter combined. Yet physicists still do not know what it is. It might be an energy belonging to empty space itself, or something stranger that changes over time.

Key facts

Share of the universe: ≈ 68–69%Of the total energy content (Planck 2018)
Discovered: 1998From distant Type Ia supernovae
Nobel Prize in Physics: 2011Perlmutter, Schmidt and Riess
Acceleration began: ≈ 5–6 billion years agoBefore that, gravity was slowing expansion
Leading explanation: Cosmological constantEnergy of empty space, written Λ
Hubble constant: ≈ 67–73 km/s/MpcMethods disagree: the 'Hubble tension'
Density: Extremely lowRoughly the mass-energy of a few hydrogen atoms per cubic metre
Key surveys: DESI, EuclidMapping expansion history (as of 2025)

An expanding universe

In 1929 Edwin Hubble, building on work by Georges Lemaître and others, showed that galaxies are receding from us at speeds proportional to their distance. This is not galaxies flying through space away from a centre, but space itself stretching everywhere, carrying galaxies apart. Astronomers describe this with a 'scale factor' that measures how much larger the universe has become over time.

Gravity from all the matter in the universe pulls against this expansion, so the natural expectation was that it should be gradually slowing. The big question in the 1990s was by how much, which would reveal whether the universe would expand forever or eventually collapse. Measuring the slowdown meant comparing the expansion rate today with its rate billions of years ago.

Reading the explainer

In the 3D view, galaxies drift apart while a graph traces the scale factor over time. Early on, the curve bends downwards as gravity slows the expansion. Later, as matter thins out and dark energy takes over, the curve bends upwards: the expansion begins to accelerate.

The 1998 supernova discovery

Two rival teams, the Supernova Cosmology Project led by Saul Perlmutter and the High-Z Supernova Search Team led by Brian Schmidt with Adam Riess, set out to measure the slowdown using Type Ia supernovae. These explosions of white dwarf stars reach a similar, predictable peak brightness, so how bright they appear reveals how far away they are. Their light also shows how much the universe has stretched since it left them.

Both teams found that distant supernovae were fainter than expected. They were farther away than a slowing universe would allow, which meant the expansion had sped up. The result was so surprising that each team checked it extensively, and the agreement between them helped convince the field. Perlmutter, Schmidt and Riess shared the 2011 Nobel Prize in Physics for the discovery.

What could dark energy be?

The simplest explanation is the cosmological constant, a term Albert Einstein added to his equations of general relativity in 1917 and later abandoned. It represents a fixed energy density belonging to space itself. As the universe expands and creates more space, the total amount of this energy grows, while matter becomes more spread out. Eventually the push of dark energy outweighs the pull of matter, and the expansion accelerates.

The trouble is that quantum physics predicts an energy of empty space vastly larger than the value observed, by many tens of orders of magnitude. This mismatch is one of the deepest puzzles in physics. Alternatives include a slowly changing energy field, sometimes called quintessence, or the possibility that general relativity itself needs modifying on the largest scales.

The cosmic recipe

Combining supernovae, the cosmic microwave background and the clustering of galaxies gives a consistent picture known as the Lambda-CDM model. In it, the universe today is roughly 68–69% dark energy, 26–27% dark matter and about 5% ordinary matter. Dark energy has only outweighed matter for the last few billion years.

The Hubble tension and hints from DESI

The current expansion rate, the Hubble constant, comes out at about 67–68 kilometres per second per megaparsec when predicted from the cosmic microwave background, but closer to 73 when measured directly from nearby supernovae and pulsating stars. This disagreement, called the Hubble tension, has persisted as measurements have improved. It may point to unrecognised errors, or to physics missing from the standard model.

The Dark Energy Spectroscopic Instrument (DESI), on a telescope at Kitt Peak in Arizona, is mapping millions of galaxies and quasars to trace the expansion history in detail. Its results released in 2024 and 2025 hint that dark energy may have weakened over time rather than staying constant. The evidence is suggestive but not yet conclusive, and depends partly on which other data sets are combined with it. If confirmed, it would mean dark energy is not a simple cosmological constant.

The fate of the universe

If dark energy stays constant, the expansion will keep accelerating forever. Galaxies beyond our local group will slowly recede beyond view, and in the far future observers in the Milky Way, by then merged with Andromeda, would see an almost empty sky beyond their own galaxy. Stars would eventually burn out, leaving a cold, dark universe.

If dark energy changes, the story could differ. A strengthening dark energy could, in principle, tear structures apart in a 'Big Rip', while a weakening one might allow the expansion to slow again. Surveys such as DESI, ESA's Euclid, the Vera C. Rubin Observatory and NASA's Nancy Grace Roman Space Telescope aim to pin down its behaviour precisely enough to tell these possibilities apart.

Surprising facts about Dark Energy

Dark energy is the largest component of the universe, yet in a volume the size of Earth it would amount to only a tiny fraction of a gram of mass-energy.
Both teams that discovered cosmic acceleration originally set out to measure how fast the expansion was slowing down.
Einstein introduced the cosmological constant to keep the universe static, and decades later it returned as the leading explanation for acceleration.
For roughly the first 8 billion years of cosmic history, the pull of matter won and the expansion was decelerating.
Light from the most distant galaxies we see today left them when they were much closer to us, and the space in between has since stretched enormously.

Frequently asked questions

What is dark energy in simple terms?: Dark energy is the name for whatever is making the universe expand faster and faster. It seems to be spread evenly through space and makes up about 68% of the universe's energy content, but its true nature is unknown.
How do we know the universe's expansion is accelerating?: Distant Type Ia supernovae appear fainter, and therefore farther away, than they would be if expansion were slowing. This result has since been backed up by the cosmic microwave background and maps of how galaxies cluster across cosmic time.
Is dark energy changing over time?: Possibly. Results from DESI in 2024 and 2025 hint that dark energy may have weakened slightly over billions of years, but the evidence is not yet strong enough to be certain. Further data from DESI, Euclid and other surveys should settle the question.
What is the Hubble tension?: It is a disagreement between two ways of measuring the universe's current expansion rate. Predictions from the early universe give about 67–68 km/s per megaparsec, while direct local measurements give about 73, and the gap has not gone away as measurements improved.
Will dark energy rip the universe apart?: Not if it behaves like a cosmological constant, which current data broadly favour. In that case galaxies, stars and planets stay bound together while distant galaxies drift out of sight. A 'Big Rip' would require dark energy to grow stronger over time.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources