Dark Energy Explained: The Force Making Up 68% of the Universe — and Why We Have Absolutely No Idea What It Is

The universe is expanding. That's been known for a century.

What nobody expected — what genuinely stopped two teams of astronomers cold in 1998 — is that it's speeding up.

Not coasting. Not gradually slowing. Accelerating. Getting faster every second of every day. Something is pressing against every galaxy in existence, and the more space it creates, the stronger it gets.

We call it dark energy. It accounts for 68% of everything in the universe. Every satellite, every star, every ocean, every living creature — that's the other 5%.

Every satellite in SkyLens's live tracker — every spacecraft, every piece of debris, every astronaut in orbit right now — is part of that 5%. The thin, visible sliver of reality we've actually managed to study.

The other 95% still doesn't make sense.

What is dark energy in simple terms?

Dark energy is a pressure built into the fabric of space itself. Not a particle, not a wave, not a field you could detect in a laboratory. It's a property of emptiness. Every cubic metre of void contains it. And here's what makes it strange: the more space expands, the more dark energy there is — which drives more expansion — which creates more dark energy.

A self-fuelling loop, running since before Earth existed. It will keep running until there is nothing left to see.

How do we know dark energy exists?

In 1998, two teams of astronomers were racing to answer what seemed like a settled question: how quickly is the universe's expansion slowing down? Gravity was supposed to be pulling everything back. They just wanted the number.

Their tool was the Type Ia supernova — a specific kind of stellar explosion so consistent in peak brightness that it works as a cosmic yardstick. Measure how bright one appears from Earth, and you know exactly how far away it is. Measure its redshift, and you know how fast it's moving away. Straightforward physics. Settled science.

The answer came back impossible.

The distant supernovae were dimmer than they should have been — meaning they were farther away than gravity could explain. The expansion wasn't decelerating. It was accelerating. Something was pushing the universe apart, and it had been doing so for billions of years.

Both teams assumed they'd made an error. They checked the data. They checked again. They hadn't made an error.

Saul Perlmutter, Brian Schmidt, and Adam Riess shared the 2011 Nobel Prize for the discovery. In his Nobel lecture, Riess described how both teams spent months trying to explain the result away before accepting what the data was actually saying. You can explore how space observation works on SkyLens.

Did Einstein predict dark energy?

Sort of. By accident. Then he erased the prediction in embarrassment. Then he turned out to be right.

In 1917, Einstein was applying general relativity to the whole cosmos. His equations kept insisting the universe couldn't be static — it had to be either expanding or contracting. Since the prevailing view was that the universe sat still and eternal, he added a fudge factor to make the math cooperate: the cosmological constant, represented by the Greek letter Λ (Lambda). A built-in counter-pressure to prevent cosmic collapse.

Then Hubble proved the universe was expanding. Einstein deleted the cosmological constant and reportedly called it his "greatest blunder." Case closed.

Seventy years later, dark energy turns up — and it behaves almost identically to the cosmological constant Einstein threw away. To be fair: some physicists argue the two aren't precisely the same thing, since the quantum mechanical prediction for vacuum energy is wildly different in magnitude from what we actually observe. But functionally, the term Einstein called his greatest blunder remains our best working description of the dominant force in the universe.

What will dark energy eventually do to the universe?

Right now, dark energy and gravity are in a slow-motion tug of war. Locally, gravity wins — it holds solar systems together, keeps galaxies intact, anchors you to the surface of Earth. But on cosmic scales, dark energy is winning. And over time, it wins more.

In roughly 100 billion years, every galaxy outside our local neighbourhood will have moved so far away that its light can never reach us again. The sky goes dark. Permanently.

A civilisation born then would look up and see only the Milky Way. No other galaxies. No evidence the wider universe ever existed. The entire history of the visible cosmos, erased by distance.

Is dark energy actually a constant?

This is the question keeping cosmologists tense right now — and recent data has started to shake the foundations of the standard model.

Einstein's cosmological constant assumes dark energy is fixed: same strength everywhere, at all times, forever. But in April 2024, the Dark Energy Spectroscopic Instrument (DESI) — a spectrograph in Arizona that mapped over 6 million galaxies in its first year of operation — released results suggesting dark energy might be changing over time. Getting stronger. Or weaker. Something other than a fixed constant.

The DESI team was carefully cautious: the statistical hint sat below the threshold physics requires for a confirmed discovery. It could still be a fluctuation in the data. However, when the DESI signal is combined with data from other cosmological surveys, the hint strengthens — and several leading cosmologists have called it the most significant indication of new physics in decades.

ESA's Euclid telescope (already collecting data) and NASA's Nancy Grace Roman Space Telescope are both in operation specifically designed to chase this question. If dark energy is evolving, they'll confirm it within the decade. If they do, every prediction we have about the universe's ultimate fate needs to be rewritten from scratch.

Why does the size of this mystery actually matter?

Dark energy isn't just a gap in our knowledge. It's the centrepiece of what physicists call the cosmological constant problem: quantum field theory predicts that empty space should be seething with enormous energy. The actual measured value of dark energy is roughly 10¹²⁰ times smaller than that prediction.

Not a rounding error. The largest known mismatch between theoretical prediction and physical measurement in the history of science. Something fundamental about how we understand energy, space, and quantum mechanics must be wrong — and no one knows what.

Some physicists argue this points toward new physics entirely beyond the Standard Model. Others argue the calculation is flawed — that we're misapplying quantum field theory to gravity in ways that artificially inflate the mismatch. Both sides have serious proponents. Neither has been confirmed.

For now, dark energy remains the most dominant and least-understood force in existence. 68% of everything. Zero explanation. Follow more deep-dives into the universe's biggest open questions at the SkyLens blog.