What is dark matter?

The Universe consists mostly of the unknown substances dark matter and dark energy, but what exactly are they?

We explore why scientists think the invisible exists, and the theories that try and decipher these unexplained mysteries.

Written by Nadia Blackshaw

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What is dark matter? This image shows its distribution in the centre of the Abell 1689 galaxy cluster. The image was taken by the Hubble Space Telescope and scientists added the blue areas of dark matter.
Credit:NASA/ESA/D. Coe (NASA JPL/Caltech and STScI)

 

If you’ve read alot of science news over the last decade, chances are you’ve come across dark matter and dark energy.

These mysterious terms may seem at home in a Hollywood movie, but are actually the basis of much modern-day scientific research.

The ideas are not new, although research has taken off in recent years.

Dark matter can be traced back to the 1920s and dark energy has its origins with Einstein and his theory of general relativity. And these are big players in the Universe.

Current evidence shows that dark matter makes up around 25 per cent of the Universe and dark energy the majority, at around 70 per cent.

The ‘normal’ matter we can all see and touch is only 5 per cent, showing the importance of the science behind these dark phenomena.

But with 95 per cent of the Universe consisting of them, what actually are dark matter and dark energy?

 

Dark Matter: The invisible stuff making our galaxies speed

Dark matter is the simpler of the two to explain, but this is deceptive as it’s still eluding scientists. It is ‘invisible’ to us, meaning that radiation doesn’t interact with it, although it is influenced by gravity.

“Dark matter is something mysterious and we have no clue what it’ll be until its discovered,” says Horst Fischer, researcher at the CAST (CERN Axion Solar Telescope) experiment at CERN.

The evidence that invisible matter is floating around in space mostly comes from its interaction with gravity.

From observation, we know that galaxies spin, and physics tells us that stars farther from the centre of a galaxy should move slower in their orbits.

However, galaxies we observe don’t follow this law as the outer stars move just as quickly as those closer in.

The rate at which they spin also means that the outer mass should be flung outwards into space because of this unstable orbit.

Dark matter 'fixes' the problem by adding extra unseen mass. In fact, scientists can map where dark matter clumps in galaxy formations by calculating where extra mass is needed.

 


The galaxy cluster 1E 0657-56, known as the Bullet Cluster. Red represents the total visible mass and the blue hues show the distribution of dark matter in the cluster. 
Credit: NASA/CXC/CfA/M.Markevitch et al

 

Other evidence follows from a similar argument.

Light bends due to strong gravitational forces, and by detecting how much distant light is displaced by galaxies, scientists infer more mass than can be seen with conventional telescopes.

Even galaxy clusters need a dark matter web surrounding them to explain their formation.

The result is that most scientists accept the existence of dark matter, but this leads to the question as to what is it made of. Simply, we don’t know yet.

“In about 3 cubic centimetres you would have the mass of dark matter equivalent to about the mass of a proton,” says Professor Fischer.

“It’s very dense and so it’s very surprising that we haven’t discovered dark matter yet.”

Candidates for this mysterious substance are supersymmetric particles, such as Weakly Interacting Massive Particles (WIMPs) or other hypothetical particles like axions or sterile neutrinos.

None of these has been discovered yet, but they theoretically have properties that match dark matter.

A more recent proposal theorises that dark matter ‘shapeshifts’ between phases at different size scales, from particles to a superfluid, and this is why it’s been so hard to identify.

Although an exciting hypothesis, it’s still some way from being proven.

Despite the strong evidence in favour of dark matter, a number of alternate theories exist that often need to introduce drastic modifications to general relativity in order to explain observations.

 

Dark Energy: The strange substance making space stretch

Dark matter might seem a little spooky, but it has nothing on dark energy.

Dark energy is the name we give to a hypothetical field that seems to permeate through space, triggering an acceleration in the Universe’s expansion.

In other words, dark energy is causing space itself to stretch.

Gravity is an attractive force, and with mass dotted throughout space, many scientists initially believed that the Universe would eventually stop expanding under the gravitational pull exerted by massive bodies.

However, in the late 1990s, observations of Type Ia Supernovae showed the opposite - the expansion rate was accelerating. This suggests a strange effect is in action.


 

“The Universe is not completely smooth; there’s more mass over here than over there and that lumpiness changes over time because of gravity,” says Josh Frieman, director of the Dark Energy Survey and staff scientist at Fermilab.

“Looking at the evolution of that lumpiness over cosmic time is a way of giving us a handle on the cosmic tug of war between dark energy, which is pushing stuff apart, and gravity, which is pulling stuff together.”

Theories of dark energy are diverse. The simplest harks back to Einstein and his cosmological constant.

It was included in the theory of general relativity to balance gravity and maintain a static Universe (one that is neither expanding nor contracting), but Einstein abandoned it when the American astronomer Edwin Hubble discovered the Universe was expanding.

As a candidate for dark energy, the cosmological constant is a constant energy field filling space evenly, but observation does not match the models.

Quantum field theory predicts a vacuum energy that could be equivalent to the cosmological constant.

However, calculations of this energy are 120 orders of magnitude higher than the experimentally observed effect; a colossal difference scientists need to explain.

Others believe dark energy changes over time; that it’s not a constant effect.

This dark energy field is sometimes referred to as ‘quintessence’ and could be a lighter cousin of the Higgs boson.

This theory, depending on how ‘quintessence’ evolves, could predict how the Universe will end.

Hypotheses that try to exclude dark energy often rely on modified theories of gravity.

These can be worked through mathematically but can fall short of explaining all the observed phenomena.

Some theories even postulate that the acceleration of expansion is an illusion, due to the relative motion or lower densities of space in our local galaxy cluster.

“More generally, we use this term dark energy a lot but we actually don’t even know if dark energy is causing the universe to speed up,” says Professor Frieman.

“It could be that we don’t understand gravity. That something happens to gravity on the larger scales of the Universe that changes its behaviour, and that could be what gives rise to cosmic acceleration.”

 

Are we just floundering around in the dark?

There is a lot of work left to do to fully understand dark matter and dark energy.

Although often discussed as distinct substances, there are people looking into possible links between the two.

Maybe dark matter decays into dark energy, or both have a common origin. They could even be manifestations of faster-than-light particles called tachyons.

Dark matter might tell us how the Universe began and its fate could be decided by dark energy.

Either way, you will most definitely be hearing about both dark matter and dark energy in the future, hopefully with a bit more light shed on them.

 


Carousel Image: Artist's impression of dark matter web in the Universe
Credit: NASA
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