Our world is an afterthought in a universe ruled by something far darker
In classical times, everything in the "sublunary sphere" (the physical universe beneath the Moon) was widely believed to consist of the four "elements" – earth, air, fire and water. But there was also curiosity about what lay beyond . At the time, the cosmos was thought to be made from a more perfect "fifth essence".
We've since learnt that everything on Earth is made of atoms – the 92 naturally occurring elements of the periodic table – and, more remarkably, that stars and nebulae are all made up of similar atoms.
Yet as we've compiled a more complete inventory of what's out in space , something very surprising has emerged. There seems to be at least five times more stuff in the universe that takes some form other than atoms – this is the mysterious "dark matter" .
Unfortunately for scientists, it happens to be invisible through a telescope. But it is a convincing hypothesis that it is there. If there weren't so much of this dark stuff (including immaterial energy – latent in empty space itself), galaxies would not be stable but would fly apart. The beautiful discs or spirals we can see through telescopes are essentially "luminous sediment" held in the gravitational clutch of an all-pervasive swarm of invisible particles of entirely unknown nature.
So, it is little wonder that scientists have been working to discover more about what this "dark matter" might be. There is currently excitement that an international group working in a mine in South Dakota has detected a "flash" that could be caused by a dark matter particle crashing into a xenon atom .
Researchers working in a mine in South Dakota believe they may have detected dark matter using a highly sensitive machine called the Lux-Zeplin - Matthew Kapust/Reuters/Sanford Underground Research Facility
I'm sceptical of this interpretation. But the scientific stakes are so high that this will rightly incentivise further work, not least because the "dark side" seems to control our universe – how it began, its eventual fate and whether it's finite or infinite.
There are no very convincing theoretical arguments about how heavy each particle of "dark matter" might be: best guesses have suggested no more than a few hundred times as heavy as a hydrogen atom.
If there were enough such particles to make up all the dark matter in our galaxy, there would then be several thousand per cubic metre in the neighbourhood of the Sun. They would be moving at about the same speed as the average star in our galaxy – maybe 185 miles per second. The dark matter particles would generally whizz straight through the Earth.
However, a tiny proportion might interact with an atom in the material they pass through. Very sensitive experiments, such as the one in South Dakota, are seeking to detect the minuscule "kick" or recoil from these rare impacts. The experiment must be cooled to a very low temperature, and carried out deep underground to reduce the confusion from other kinds of events that could drown out any genuine signal. Several groups of physicists have taken up the challenge of this "underground astronomy".
It's delicate and tedious work, but if these "dark matter" experimenters do eventually succeed, they will not only find out what our universe is mainly made of, but as a bonus may discover an important new kind of particle. So far there have been no firm detections, despite the excitement this week. If you are seeking fish in a lake and catch nothing, it could be that the mesh of your net isn't fine enough.
The spirals we see through telescopes are 'luminous sediment' held together by invisible dark matter - E+
Even if the results of these investigations may have been disappointing so far, the search itself remains important. These dark matter searches forge new links between the very large and the very small – the "inner space" of the subatomic world and the "outer space" of the cosmos. They exemplify how the cosmos offers a chance for us to learn from "natural" experiments that we could never do here on Earth.
There's a symbiosis between cosmology and laboratory physics. Astronomers obviously need to know how atoms behave in order to understand the stars – they learn this via laboratory experiments. But now, in a complementary way, astronomers are discovering things that tell physicists something new. We can see where nature has done experiments for us – galaxies crashing together, stars exploding, immensely strong gravity.
It is also another step on the pathway humanity has taken since the time of the ancients. We've gradually learnt, ever since Copernicus, that we're not located at any special central place in the cosmos .
Now it turns out that we're not even made of the dominant stuff – atoms seem a mere afterthought in a universe whose structure and fate are controlled by quite different and mysterious substances. It is a humbling thought, and a spur to new discoveries.
Martin Rees is the former Astronomer Royal of the United Kingdom. His new book, Secrets of the Cosmos, will be published by Penguin in January 2027
