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What is dark matter?

The Standard Model of particle physics has one gaping hole; it fails to account for around 95% of the universe.

Dark matter is one of those subjects that illustrates just how spectacularly ignorant we are about the world we live in.

Stars in space © NASA

NASA’s 'direct proof' of dark matter (coloured blue – inferred by gravitational effects)

Fritz Zwicky and blackboard with equations
© Science Museum

Missing matter

The Standard Model of particle physics, the theory that describes all the known particles, and hence everything that makes up the visible universe, is considered one of mankind’s greatest intellectual achievements. A triumph of our ability to make sense of nature’s apparent chaos and uncover the basic laws that operate at the roots of reality.

And yet despite its success in describing the stuff that we are made from, the Standard Model has one gaping (and rather embarrassing) hole. It fails to account for around 95% of the universe.

In fact, it was thanks to astronomers in the 1930s that the extent of our knowledge about the contents of the universe was exposed. In 1933, Fritz Zwicky noticed that galaxies in the distant Coma cluster appeared to be moving too fast for the gravity generated by the visible stuff (stars and gas) to hold them within their cluster – a vast collection of galaxies.

Later, it was noticed that stars in the nearby Andromeda galaxy were also whizzing about at such a pace that they should fly off into intergalactic space, rather than carry on obediently circling the galactic centre.

As the evidence mounted, astronomers were forced to a staggering conclusion: there was something else out there. This something had to be completely invisible and, even more remarkably, it had to be about four times as abundant as ordinary visible matter. It was this enigmatic so-called “dark matter” that created the gravity necessary to keep the stars aligned.

Recent results from ESA’s Planck Satellite, which was launched in May 2009 to map the cosmic microwave background (a faint remnant of light from the superhot fireball that filled the universe 380,000 years after the Big Bang), show dark matter makes up 26.8% of the stuff in the universe, with ordinary matter accounting for just 4.9%. The remaining 68.3% is the even more mysterious 'dark energy', the cause of an unknown force which is apparently causing the cosmos to expand at an ever increasing rate.

Snapshot of the oldest light in the universe © ESA / Planck Collaboration

The Planck Map, a snapshot of the oldest light in the universe when it was just 380,000 years old.

Large detectors from Cern
The LHC’s CMS detector. © CERN

Searching in the dark

One thing we know for sure is that this dark matter isn’t made of atoms – if it was it would interact with light and be visible (hence the name 'dark'). It must be made of some altogether different thing, presumably a new particle outside of the Standard Model.

So how do we figure out what this stuff is? There are a range of different experiments of varying size and complexity designed to detect particles of dark matter.

Some involve burying extremely sensitive detectors deep underground, shielded from constant bombardment by cosmic rays, which watch patiently for the tiny flashes of light expected if a particle of dark matter happens to bump into the nucleus of an atom.

Others use gigantic particle accelerators like the Large Hadron Collider (LHC) that hope to directly produce particles of dark matter by smashing more ordinary particles together.

The real problem with dark matter is that it doesn’t interact except though the weak force. As its name suggests, the weak force is weak, only noticeable at extremely short distances, which means a particle of dark matter has to bump directly into the nucleus of an atom to produce a noticeable effect.

When you consider that atoms are mostly empty space, with the nucleus the proverbial “fly in the cathedral”, such an event is vanishingly unlikely. As a result, particles of dark matter can happily float through the entire earth and out the other side without so much as shaking hands with a single atom.

Galaxy cluster in space
NASA’s Hubble Space Telescope image of a galaxy cluster showing gravitational effects caused by dark matter. © NASA / ESA

The real problem with dark matter is that it doesn’t interact except though the weak force. As its name suggests, the weak force is weak, only noticeable at extremely short distances, which means a particle of dark matter has to bump directly into the nucleus of an atom to produce a noticeable effect.

When you consider that atoms are mostly empty space, with the nucleus the proverbial “fly in the cathedral”, such an event is vanishingly unlikely. As a result, particles of dark matter can happily float through the entire earth and out the other side without so much as shaking hands with a single atom.

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