What is dark matter in simple terms? New map of the night sky explained – and what it means for science

Dark matter, dark energy and antimatter remain some of the deepest mysteries in modern physics

Researchers have created the largest ever map of dark matter, invisible material thought to account for up to 85 per cent of the total matter of the universe.

A team co-led by UCL researchers as part of the international Dark Energy Survey (DES), used artificial intelligence to analyse images of 100 million galaxies, looking at their shape, spots of light made up of 10 or so pixels, to see if they have been stretched.

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Researchers photographed the night sky using a 570-megapixel Dark Energy Camera on the 4-metre Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile (Photo: Reidar Hahn/Fermilab VMS/PA Media)

As matter curves space-time, astronomers are able to map its existence by looking at light travelling to Earth from distant galaxies. If the light has been distorted, this means there is matter in the foreground, bending the light as it comes towards us.

The new map – which is described in a new paper posted on the DES website and to be published in the Monthly Notices Of The Royal Astronomical Society – is a representation of all matter detected in the foreground of the observed galaxies, but covers just a quarter of the southern hemisphere’s sky.

But what is dark matter?

Here is everything you need to know.

The map superimposed on an image of the Milky Way showing areas of dark matter - light areas show regions where dark matter is most dense and correspond to superclusters of galaxies, while the almost black patches are large empty spaces in between the clusters (Image: N. Jeffrey/Dark Energy Survey collaboration/PA Media)

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What is dark matter?

For decades astronomers and scientists have suspected there is more material in the universe than we can see, estimating that up to 85 per cent of the mass in the universe is effectively invisible.

This dark matter cannot be observed directly because it does not interact with light in the same way as the ordinary matter that makes up stars, planets and life on Earth.

So in order to measure what cannot be seen, scientists analyse dark matter’s effect on gravity.

Dark matter, like dark energy, remains mysterious, but its existence is inferred from galaxies behaving in unpredicted ways, for instance, the fact that galaxies stay clustered together, and that galaxies within clusters move faster than expected.

Pol Gurri, a PhD student at Swinburne University of Technology, Australia, said: “It’s like looking at a flag to try to know how much wind there is – you cannot see the wind but the flag’s motion tells you how strongly the wind is blowing.”

But while scientists have observed the gravitational effects of dark matter for decades, its true nature still remains a mystery.

Questions around dark matter remain some of the deepest mysteries in modern physics, including what it is made of, and the imbalance of matter and antimatter in the universe.

These mysteries cannot be explained with the Standard Model, the guiding theory of particle physics which describes all the known fundamental particles that make up our universe and the forces that they interact with.

Some theories suggest the makeup of dark matter must be very cold and heavy, while others say it is composed of tiny particles which are so light that they behave like quantum matter and move in waves.

How was the study conducted?

Using artificial intelligence methods to study photographs from the Dark Energy Camera in Chile, researchers were able to reveal the shapes of hundreds of millions of distant galaxies.

Co-author Professor Ofer Lahav, UCL Physics and Astronomy, chairman of the DES UK consortium, said: “When we look at the night sky, we see the galaxy’s light but not the surrounding dark matter, like looking at the lights of a city at night.

“By calculating how gravity distorts light, a technique known as gravitational lensing, we get the whole picture, both visible and invisible matter.

What does the new map mean for science?

The study brings scientists closer to understanding what the universe is made of and how it has evolved.

Co-lead author Dr Niall Jeffrey, UCL physics and astronomy, said: “Most of the matter in the Universe is dark matter. It is a real wonder to get a glimpse of these vast, hidden structures across a large portion of the night sky.

“In our map, which mainly shows dark matter, we see a similar pattern as we do with visible matter only, a web-like structure with dense clumps of matter separated by large empty voids.

“Observing these cosmic-scale structures can help us to answer fundamental questions about the universe.”

The analysis suggests matter is distributed throughout the Universe in a way that is consistent with predictions in the standard cosmological model, the best current model of the Universe.

Researchers also found hints that, as with previous surveys, the Universe may be a few per cent smoother than predicted, a prediction that comes from analysis of the light left over from the Big Bang.

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