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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.

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.

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