Since ancient times, human beings have believed the night sky might hold the key to our existence. We gave names to the planets and the distant stars and dreamed of setting foot on the Moon or MarsThe fourth planet from the Sun and the second-smallest planet in the Solar System. .

And yet for hundreds of years the universeEverything. Including all of time and space and its contents. has toyed with us, yielding up some of its secrets only to perplex us with an even tougher poser.

Three years ago, a new Dark Energy Spectroscopic Instrument (DESI) was created in ArizonaA state in the southwest USA with a population of more than 7 million. to answer one of the most complex of these questions: dark energyAn unknown form of energy causing the expansion of the universe to accelerate.. And this spring, it is due to revolutionise what we know about this elusive phenomenon - or perhaps tell us that we really do not know anything at all.

Dark energy started as nothing more than a scientific theory to solve the big headscratcher of 20th Century physics. By this time it was well known that the universe was expanding. 

However, traditional NewtonianRelated to the work of Sir Isaac Newton, the English physicist and mathematician who is known for his discovery of gravity. ideas would suggest that this expansion should begin to slow under the effects of gravityAn invisible force that pulls objects towards each other. Earth's gravity keeps us on the ground and makes things fall. It is less strong on Mars., as it did for the first 10 billion years after the Big BangThe moment the universe came into being. It is thought that this took place in a kind of explosion from a dimensionless singularity.. However, astronomers observed that for the last four billion years, it has been expanding more and more quickly.

Scientists thought there must be something in the universe capable of exerting a vast gravitational effect in the opposite direction to ordinary matter. They gave it the name "dark energy". Physicists calculate that fully 68% of the density of the universe is made up of it.¹

It is unlikely we can do anything useful with dark energy, as the amounts we would theoretically be able to harness would be too small.² However, understanding it is key to predicting the fate of our universe.

If the universe continues to expand, then it will eventually suffer "heat death": a state where all energy and matter are so widely distributed that most complex reactions will be unable to take place. Physics will simply cease to take place.³

More dramatically, dark energy could come to be the dominant force all over the universe and begin to tear through ordinary matter, even breaking down atoms themselves. This outcome is colloquiallyLanguage that is used in ordinary conversations. Informal language. known as the "big rip".⁴

But between now and 2030 we might yet discover still stranger things about dark energy. Scientists always thought that since the universe expands at the same rate in all directions, this energy must be the same everywhere, uniformly filling all empty space.

However, preliminary results from DESI suggest this might not be true. Instead, it seems that dark energy might have changed over time, becoming weaker today than it was in the past.

One group of scientists has an even more radical theory: that dark energy does not really exist. They argue the universe is expanding not homogeneouslyActing all in the same way. but in a "lumpy" way, more in some places than others.⁵

We have long known that time goes faster in empty space than it does when surrounded with matter. What this means is that emptier parts of the universe have experienced billions more years than we have.

We might expect it to have expanded further in that time, accelerating the pace of overall expansion without any need for dark energy.

In other words, everything we thought we knew for half a century could be wrong. Will 2030 prove us wronger still?

Will we be astonished by how little we know now?

Yes: So far both measurements taken by DESI and more theoretical work are overturning everything we thought we knew about dark energy in 2025. By 2030 we might need an all-new theory.

No: The DESI findings are very speculative and few scientists accept the idea that dark energy does not exist. By 2030 we will probably have confirmed everything we thought was true.

Or... The secrets of the universe are waiting to be discovered. But political, social, and climatic events here on Earth may yet prevent us from ever uncovering them.

Mars - The fourth planet from the Sun and the second-smallest planet in the Solar System.

Universe - Everything. Including all of time and space and its contents.

Arizona - A state in the southwest USA with a population of more than 7 million.

Dark energy - An unknown form of energy causing the expansion of the universe to accelerate.

Newtonian - Related to the work of Sir Isaac Newton, the English physicist and mathematician who is known for his discovery of gravity.

Gravity - An invisible force that pulls objects towards each other. Earth's gravity keeps us on the ground and makes things fall. It is less strong on Mars.

Big Bang - The moment the universe came into being. It is thought that this took place in a kind of explosion from a dimensionless singularity.

Colloquially - Language that is used in ordinary conversations. Informal language.

Homogeneously - Acting all in the same way.