Science › Big Bang & cosmos
Dark energy
1998· Open question
Well supportedStrong evidence or math, but not fully tested.
An unknown property of space that is making the universe's expansion speed up rather than slow down.
See it on the Science map →How it works
Gravity should slow the expansion. In 1998, two teams measuring distant exploding stars found the opposite: for the last five billion years or so, expansion has been accelerating. Whatever causes this is called dark energy. It makes up about 68% of the universe's contents. The simplest explanation is energy built into empty space itself, matching a term Einstein added to his equations in 1917 and later dropped.
Why it matters
It dominates the universe and will set its long-term fate. It is perhaps the biggest gap in physics: most of reality is something we cannot name.
How we know
Supernova studies by teams led by Saul Perlmutter, Brian Schmidt and Adam Riess (Nobel 2011). Confirmed by galaxy-clustering patterns and the microwave background.
Still a mystery
What is it, and is it constant? Results from the DESI survey in 2024-2025 hint it may be weakening over time. Not yet conclusive.
Connections (6)
▸Dark energy is speeding up The universe is expandingWell supported
The relationship
Gravity from matter slows expansion. Dark energy pushes the other way, and as space grows, matter thins out while dark energy apparently does not. About six billion years ago dark energy's push overtook matter's pull, and expansion switched from slowing down to speeding up. If it stays constant, galaxies beyond our local group will eventually drift out of view.
How the link was found
Two teams measuring exploding white-dwarf stars, led by Saul Perlmutter and by Brian Schmidt with Adam Riess, announced acceleration in 1998 (Nobel 2011).
Is it proven?
Confirmed independently by patterns in galaxy clustering and by the microwave background. That expansion is accelerating is well supported; what drives it is unknown.
Open questions and mysteries
DESI survey results in 2024-2025 hint dark energy may be weakening. If so, the long-term future of expansion changes.
▸The quantum vacuum isn't empty predicts about 10^120 times too much Dark energyOpen question
The relationship
Empty space, in quantum theory, holds fluctuation energy, and energy in empty space would push space apart exactly as dark energy does. So vacuum energy is the obvious explanation. But adding up the fluctuations naively gives a value about 10^120 times larger than observed. Something cancels almost all of it, or the reasoning is wrong.
How the link was found
Yakov Zel'dovich connected vacuum energy to Einstein's cosmological constant in 1967. Steven Weinberg's 1989 review made the mismatch famous.
Is it proven?
Not proven. Vacuum effects on atoms and metal plates are measured, but no one has measured how vacuum energy gravitates. The 10^120 gap is a calculation, often called physics' worst prediction.
Open questions and mysteries
Why is the vacuum's energy so tiny but not zero? This may require quantum gravity, a multiverse, or a new idea entirely.
▸Dark energy will decide How the universe endsWell supported
The relationship
Matter thins out as space grows, so the future rides on how dark energy's density changes. If it stays constant, as with a cosmological constant, expansion accelerates forever into a cold, thinning universe. If it grows, it eventually overwhelms every bond, from galaxies to atoms: a Big Rip. If it fades and turns negative, gravity could win and the universe recollapse in a Big Crunch.
How the link was found
Robert Caldwell, Marc Kamionkowski and Nevin Weinberg described the Big Rip in 2003. Recollapse scenarios gained renewed attention after DESI's 2024–25 results.
Is it proven?
Dark energy has outweighed matter for about the last 3.5 billion years, and matter keeps thinning, so in the Friedmann equations dark energy's behavior controls the far future. That logic is solid; which ending follows depends on measurements still in progress.
Open questions and mysteries
Forecasts beyond a few billion years extrapolate a quantity whose nature is unknown. Even the direction of its long-term trend is uncertain.
▸1998: exploding stars reveal faster expansion revealed Dark energyProven
The relationship
Acceleration means something is pushing space apart, yet ordinary matter, dark matter and light can only pull. The push requires a new ingredient with strongly negative pressure. The supernovae, combined with measurements of matter, implied it made up about 70% of the universe's energy, more than all matter combined. The name 'dark energy' came the same year.
How the link was found
Michael Turner coined 'dark energy' in 1998, by analogy with dark matter. The supernova papers came from Riess and colleagues (1998) and Perlmutter and colleagues (1999).
Why it's proven
The afterglow shows flat space, which needs about three times more energy than matter supplies; galaxy surveys weigh the matter independently. The gap matches the supernova amount.
Open questions and mysteries
Dark energy is a name for an effect, not an explanation. Is it energy of empty space, a new field, or a sign that general relativity needs correcting on cosmic scales?
▸The cosmological constant (Λ) is the simplest explanation of Dark energyWell supported
The relationship
In general relativity, pressure gravitates as well as mass. Ordinary pressure adds pull; strong enough negative pressure reverses it into push. A cosmological constant's pressure is exactly the negative of its energy density, which is also what keeps its density fixed as space grows. That is just what dark energy looks like so far, and it needs only one number, which Einstein's equations already allowed.
How the link was found
Before the supernovae, Lawrence Krauss and Michael Turner, and separately Jeremiah Ostriker and Paul Steinhardt, argued in 1995 that the data already favored a cosmological constant.
Is it proven?
Until 2024, all measurements of dark energy's pressure-to-density ratio gave about −1, the constant's value, within a few percent. DESI's hint is the first notable departure.
Open questions and mysteries
If it is the constant, why is its value so small? If it is not, what replaces it? Either answer likely needs physics beyond what we know.
▸Dark energy may instead be Quintessence: dark energy that changesSpeculative
The relationship
A field's energy has two parts: motion, and position on its energy hill. When the field barely moves, position energy dominates, and that gives negative pressure, the same trick that powered inflation, at a vastly lower energy. Dark energy would then be a field still creeping downhill. Its push weakens as it picks up speed, so its strength would differ between eras.
How the link was found
Bharat Ratra and Jim Peebles, and Christof Wetterich, independently in 1988. Ivaylo Zlatev, Limin Wang and Paul Steinhardt (1999) showed some fields could 'track' matter, perhaps explaining why dark energy took over so recently.
Is it proven?
Not proven. To tell it from a constant, surveys must measure the expansion history to about 1% at many eras. Only DESI's hint so far favors change.
Open questions and mysteries
Even if dark energy rolls, why is its energy so small? Quintessence explains change, not size; the vacuum-energy puzzle remains untouched.