Mysterious Universe: An Introduction to Dark Matter

23 October 2017

What is dark matter and why are scientists so eager to find it?

Ninety five per cent of the universe is missing.

We can’t see it; we don’t know what it’s made of; and we’re not even entirely sure it exists, but what we do know is that there’s a great deal more to the universe than meets the eye.

Planets, stars, asteroids, galaxies – the things that we can actually see – constitute less than 5% of the total universe. So what makes up the rest? Therein lies the mystery.

Research suggests that a large proportion of the universe is composed of a strange substance known as ‘dark matter’.

Dark matter is a puzzle that has plagued scientists for almost a century, but as technology advances, we’re edging closer towards the truth. Scientists all over the world are hunting for dark matter, and the even more mysterious ‘dark energy’ – and what they find could eventually transform our very notion of the cosmos.

How do we know dark matter is real?

Dark matter is simply the name we give for all the mass in the universe that we can’t detect. So how do we know dark matter exists if we can’t detect it? Well, the short answer is that whilst we can’t see dark matter, we can see its effects.

Galaxies are ‘meshed’ together:
Scientists have worked out that the universe behaves as though it contains a great deal more ‘stuff’ than it appears to. For instance, galaxies rotate within space at vast speeds – so fast that they really ought to be thrown apart. And yet, they remain intact.

For fast-moving galaxies to stay together, they need a lot of gravitational force. All matter exerts this force – it’s what pulls objects towards one another. But as far as we can see, there simply isn’t enough in the way of planets, stars and other matter to produce the gravitational force necessary to mesh galaxies together.

And so scientists think there may be vast quantities of ‘dark’ matter – invisible to the human eye – that exerts enough gravitational force to hold everything in its place.

Space is an odd shape:
Dark matter would also help account for the phenomenon of ‘gravitational lensing’. When astronomers observe distant galaxies, they often appear distorted. Rather than a neat spiral shape, the galaxy looks stretched and oddly shaped.

This effect is the result of gravitational forces that bend the light around the galaxy. But, again, not enough mass has been accounted for by the galaxies within the astronomers’ line of sight to match the amount of gravitational lensing that takes place.

In fact, some of the best evidence that we have for the existence of dark matter are observations of the Bullet Cluster. This is actually two galaxies colliding – with gravitational lensing distorting our view of the galaxies behind them.

All this suggests that invisible, dark matter might be contributing to the gravitational force, thus making galaxies appear more distorted than they otherwise would be.

Cosmic Microwave Background Radiation (CMBR):

Around 13.8 billion years ago, the universe began with the Big Bang. This humungous event left behind ‘relic radiation’ that we can still sense today.

By studying ‘relic radiation’, also known as ‘cosmic microwave background radiation’ (CMBR), scientists can piece together a picture of what the universe looked like billions of years ago. It’s almost like looking at an old photograph or a historical map of the ancient cosmos.

Observations of the CMBR seem to suggest that the universe is much denser than it appears. It also hints at ‘hotspots’ in the universe where dark matter may be concentrated. Using this complex information, scientists have pieced together maps of the universe that estimate the locations of dark matter.

Ultimately, it is this ‘relic radiation’ – which has been drifting through the universe for billions of years – that provides our best reason yet for believing that dark matter exists.

Ruling out the alternatives

In order to work out what dark matter might be, we first need to understand what it isn’t.

It’s not ‘normal’ matter:
We know that it’s not planets or stars or galaxies because unlike the normal matter they are comprised of, dark matter doesn’t emit, reflect or absorb light; it is invisible. This makes dark matter completely different from any other substance we’ve seen before. We know it must be there, but it’s hiding in plain sight.

Or antimatter:
We also know that dark matter is not the same thing as antimatter. Antimatter particles are like mirror images of normal particles – when a normal particle collides with an antimatter particle, both are destroyed, releasing energy in the process.

But astronomers have found no evidence that dark matter behaves in the same way. If all of what we term ‘dark matter’ were, in fact, antimatter, we would see traces of destruction everywhere, and would detect the energy released from antimatter collisions all over space.

Or ‘normal’ black holes:
Finally, we know that dark matter is not simply a collection of black holes created by collapsing stars. These black holes are vortex-like regions of space, which exert a vast gravitational pull. Matter surrounding black holes is sucked in and torn apart.

Because they are so violent, black holes distort the appearance of space. No stars, planets, or anything can exist close-by; even light itself is consumed. Dark matter doesn’t behave in the same way – it appears to hold things together, rather than tear them apart. Indeed, because dark matter makes up so much of the universe, if it did behave like a black hole, space would look very different.

However, it may be possible that at least some of dark matter in the universe could be formed by ‘primordial’ black holes. These are black holes created in the first moments of our universe’s existence, which can be much lighter or heavier than 'normal' black holes.

So we have an idea of how dark matter is unique from other phenomena we’ve observed, and possibly how it formed, but there are still so many questions as to what this mysterious substance actually is.

So what could dark matter be?

That’s the million dollar question. As we have seen, dark matter doesn’t behave in the way that we expect ‘stuff’ to behave. And so in order to make dark matter make sense, we need to come up with new and imaginative ideas. There are currently two leading theories:

It could be an exotic particle:
The majority of scientists believe that dark matter is some sort of exotic particle. Our knowledge of the sub-atomic world is always evolving, and it’s entirely plausible that the effects that we call ‘dark matter’ could be caused by one or more undiscovered particles.

If the universe contained colossal amounts of these unidentified particles, this would account for the excess gravitational force that scientists have observed in space – all that gravity must be coming from somewhere; an undiscovered particle seems a likely candidate. One theory which could provide such particles is supersymmetry (SUSY). Unfortunately, no concrete evidence for this idea has been found…yet.

Or maybe just plain old gravity:
But there are also scientists who believe that dark matter is not any kind of matter at all, but rather an undiscovered property of gravity. Since the 17th century, scientists have understood gravity to have fundamental characteristics, or ‘laws’, that all matter must adhere to.

But it’s possible that our existing theory of gravity is flawed. It might be that the excess gravitational force that scientists have observed in space doesn’t need extra matter to exist. If we’ve just misunderstood the laws of gravity, then these forces could exist simply as a quirk of gravity itself, not as a result of extra, invisible matter.

As it currently stands, both of these theories remain unproven. So we need more research before we can decide, once and for all, what dark matter actually is.

That’s where cutting-edge science comes in.

Unraveling the mystery

Scientists have been hunting dark matter for a long time. Swiss-American astronomer, Fritz Zwicky, first theorised its existence back in 1933. Later, in the 1970’s, the American astronomer Vera Rubin discovered firm evidence to support the existence of dark matter. Rubin predicted we’d find dark matter itself within 10 years; but almost half a century later, we’re still looking. However that’s not to say we haven’t made progress.

Research is taking place out in space:
Since the turn of the 21st century, scientists have been using advanced detectors fitted to spacecraft that are drifting through the cosmos.

These detectors are searching for evidence of high-energy ‘cosmic rays’,  a form of energy that we would expect to be produced if two dark matter particles collided – if indeed dark matter is a particle.

In recent years, detectors fitted to the outside of the International Space Station have found evidence of unusual cosmic ray events. These events could be linked to dark matter particles, but there are many more prosaic explanations out there, and so their significance remains to be seen.

And back on Earth:
From the planet’s surface, scientists are also hunting for evidence of exotic, dark matter particles by looking for their rare interactions with atomic nuclei. But in order to detect these very small, subtle effects, we first need to screen out much bigger effects, like the everyday background radiation we experience on Earth’s surface.

Scientists have got around this problem by developing laboratories deep underground, where the effects of background noise are considerably reduced.

One such facility is Boulby Underground Laboratory, located on the North East coast of England, near the North Yorkshire Moors. At 1,100 metres below ground, Boulby is the deepest mine in Great Britain, and the perfect base from which to look for dark matter.

Other similar facilities exist across the globe, from SNOLAB in Central-Eastern Canada and SURF in South Dakota, to the SUPL under construction near Melbourne.

Scientists are also searching for dark matter at the Large Hadron Collider (LHC) in CERN. Rather than looking for particles from space, researchers there are using the LHC to look for ‘supersymmetric particles’ which could provide indirect evidence about this mysterious substance.

Dark matter has proved itself to be highly elusive, but thanks to the increasingly advanced scientific tools at our disposal, we are moving closer towards the truth.

What’s next for dark matter?

Science is relentless, and so the hunt for dark matter will continue. And it’s vital that we do keep up the search. Dark matter holds the key to all sorts of secrets about the universe. Ultimately, everything we see around us, everything we’ve ever known could have been shaped in some way by this enigma.

Amidst all the unknowns, one thing is for certain: dark matter may be good at hiding, but hopefully it’s no match for the curiosity of the human spirit. Thanks to our unrelenting drive to explore, to discover and to understand, we’ll never stop hunting for answers.

Want to learn more about dark matter?

Science and Technology Facilities Council Switchboard: 01793 442000