What is dark matter?

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Science
Posted 25 July, 2019

By Imogen Learmonth

London, UK

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A quick overview of one of the universes’ greatest mysteries.

News flash: you, everything around you, and everything that you can see or touch – all observable ‘matter’ – makes up only 5% of the universe.

Yup, that’s right, 5%. The other 95%? That’s made up of dark matter and dark energy. What’s that you ask? Well, the unfathomable truth is that nobody really knows. Whilst 5% of the universe interacts with light and with the particles that make up us (meaning that we can touch and see them) 95% is made of invisible matter and forces that we cannot see, touch, smell, or interact with – it’s made of darkness.

Obviously, this enormous gap in frontier knowledge doesn’t sit well with scientists, and so they’ve spent a heck of a lot of time trying to figure out how to classify this darkness. Given that these phenomena are responsible for the makeup of the universe, we thought it would be pertinent to run you through what they’ve found so far. We still don’t know for sure what dark matter or energy is, but we can guess…

Dark Matter

After Albert Einstein discovered his theory of gravity, one thing became certain about the universe. It didn’t make any sense.

After he figured out how to calculate the gravity of any object, he was able to approximately calculate how much gravity there should be in the universe in order to make it look the way it did. Surprisingly, by his reckoning, there was nowhere near enough ‘stuff’ in existence to hold galaxies and complex structures together. If we were just working with the gravity emitted by objects we can see, stars would only exert an unequal and vague pull on one another, likely drifting apart from each other and becoming scattered about the universe.

As it is, there is enough gravity in the universe to group stars and planets together in various formations. Hence, something we can’t see is providing extra gravity. Scientists have called this unknown substance ‘dark matter’.

As well as being able to mathematically calculate the existence of dark matter, we can also see it… kind of. Whilst it doesn’t interact directly with light itself, places with a high concentration of dark matter bend light passing nearby as they have an intense gravitational field.

In other words, despite knowing nothing about dark matter, we can be sure it exists.

Multiple theories exist for what dark matter might be, but the more concrete observations revolve around what we know it’s not. We know that dark matter is not just clouds of normal matter made up of light reflecting particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them.

We also know that dark matter is not antimatter, because we don’t see the unique gamma rays that are produced when antimatter reacts with normal matter. Lastly, we know dark matter is not made up of black holes (compact objects that violently affect their surroundings through gravity) because we’d see much more gravitational lensing (weird bending of light around the event horizon of a black hole) if this was so.

Essentially, the three things we know for sure about dark matter are:

It exists
It interacts with gravity
It’s extremely abundant

Dark Energy

Scientists approximate that dark matter makes up about 27% of the universe. For all you maths whizz’s out there, if planets and stars and us make up 5%, this means that there’s still 68% left unaccounted for.

Enter dark energy.

In 1929, Edward Hubble (of the telescope fame) examined how the wavelengths of light emitted by distant galaxies shifted towards the red end of the spectrum. He found that fainter, more distant galaxies showed more red shift (the stretching of light waves through movement, making objects appear red) than closer ones. Hubble realised that this red shift matrix could only come about if the universe was actively expanding.

People’s minds were pretty blown by this – it was likely that the effects of the big bang were still being felt, as the universe continued to grow (or ‘blow up’). It was widely assumed that this expansion would eventually slow down due to the effects of gravity. Another Hubble observation turned this theory on its head, however, when observations of a distant supernovae in the 90s showed that the universe used to be expanding at a slower rate than it is today. The expansion of the universe wasn’t slowing down, it was accelerating.

Now, as anyone whose attended a 9th grade science class knows, matter can’t be created from nothing. And wherever there is empty space in the universe, more is forming every second. If the universe is growing, something must be fuelling it. Something must be filling that space.

Space doesn’t change its properties as it expands, there’s just more of it. Hence, dark energy seems to be some kind of energy intrinsic to space, that is stronger than any other energy we know, and that acts on and in empty space to create more of it. And it’s getting stronger with time. After all, empty space has more energy than everything else in the universe combined (68%).

So, how does space acquire this energy? Einstein thought that that dark energy could be a property of empty space itself, rather than a force acting upon it. He theorised it could be a field of force similar to gravity, that simply worked in the opposite way – pushing out instead of in. He named this the ‘cosmological constant’. Unfortunately, nobody has come up for a good explanation for why the cosmological constant should be there, much less why it would have exactly the right value to cause the observed acceleration.

Another theory borrows from quantum physics, suggesting that empty space is actually not empty at all, but full of tiny particles that are constantly popping in and out of existence. But when physicists tried to calculate how much energy this would give empty space, they found that the calculation was impossible.

A final, slightly outlandish theory is that Einstein’s theory of gravity is wrong altogether. But no alternative theories as of yet have been able to calculate the properties of interacting bodies as perfectly as Einstein’s.

So, really, just like dark matter, we aren’t exactly on the precipice of finding out what dark energy really is. You could say that we’re still… in the dark.

Will we ever know?

It’s both uncomfortable and exhilarating that we essentially have no idea what 95% of our universe is made up of. Our theories about dark matter and energy are still just that: theories.

I, for one, have every confidence that we’ll make leaps and bounds towards figuring out what surrounds us during the lifetime of Gen Z. We might not figure out all the answers, but then again, where would be the fun in that?

Imogen Learmonth

Departed London, UK

My interests include social and climate justice, women’s issues, and human rights. If you\’re keen on current affairs and social change, check out my profile! Follow me on Twitter and drop me some ideas/feedback via email.

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