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.
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.
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.
Written for the Science Museum's Collider exhibition, 2013. © Creative Commons - CC BY