Dark matter: Everything you need to know (almost)
Dark matter makes up more than 26% of all the energy in the Universe, while normal matter (the stuff stars, planets and you are made of) barely makes up 5%. But what is it? Why is it invisible? And what are we doing to try to find out once and for all what it is made of?
Dark matter is ‘dark’ because it doesn’t seem to emit or absorb any kind of electromagnetic radiation, including visible light. This makes it extremely difficult for scientists to detect and study, but it is thought to make up about 85% of the mass of the universe. What this ‘missing matter’ could be is therefore one of the biggest scientific questions of our time.
All in a spin about galaxies
Dark Matter was initially suggested by Swiss astronomer Fritz Zwicky in 1933, as part of his work on the rotation of a group, or cluster, of galaxies. He already knew that the rotational speed of a galaxy cluster depends on the amount of gravity it experiences. This in turn means the speed depends on the amount of mass in the cluster’s centre, as things with more mass exert more gravity.
Zwicky estimated the mass of the cluster using the number of galaxies in the group and their brightness. When he calculated the speed of the galaxies within the cluster, he found they were moving much faster than the cluster’s mass would predict. Similarly to Oort’s findings, the galaxies were moving too fast for them to even remain in the cluster. To explain this, he suggested there must be mass he couldn’t see, contributing more gravity, which he called ‘dunkle Materie’ or ‘dark matter’.
Further evidence was found in the 1970s by American astronomers Vera Rubin and Kent Ford, who measured the rotation speeds of individual galaxies. These galaxies were found to be rotating so fast that the gravity from their visible matter was insufficient to keep the galaxies together — they should have torn themselves apart.
Measurements of other galaxies followed a similar pattern, suggesting the presence of a halo of dark matter around the centre of each galaxy.
Focusing on dark matter
Albert Einstein’s general theory of relativity, published in 1915, provided physicists with an entirely new way of thinking about gravity. It added to the theory of special relativity that he had published 10 years before. Special relativity introduced the idea that the concepts of space and time can be joined into one model — spacetime, often visualised as a piece of fabric.
General relativity used the concept to explain gravity, describing how objects ‘warp’ the fabric. Picture a basketball and a tennis ball on a bedsheet. The basketball will cause the sheet to dip more than the tennis ball, because it has more mass. It is the same with spacetime — more massive objects warp spacetime more.
This led to the idea of gravitational lensing, which describes how a beam of light travelling from a light source to an observer can be bent by matter in between. Zwicky suggested in 1937 that the effect could allow galaxy clusters to act as lenses. This wasn’t confirmed until 1979, and soon began to be used to demonstrate the existence of dark matter.
If you can measure how much a galaxy cluster bends the light behind it, you can estimate the mass of the cluster. If this is different from the mass suggested by the brightness of the galaxy, you know there’s some matter you can’t see.
Navigating the early Universe
In 2013, the Planck telescope mapped out the universe’s Cosmic Microwave Background Radiation (CMBR) — the light left-over from the birth of the universe — more precisely than ever before. The Science and Technology Facilities Council (STFC) played a key role in designing and building of Planck’s instruments at the Rutherford Appleton Laboratory, with UK astronomers heavily involved in the operation of the telescope and the analysis of the data it produced.
Planck’s readings allowed researchers to measure tiny variations in the temperature of the radiation, and its map of the CMBR only really made sense when dark matter was included. This provided further evidence of its existence and helped refine our values of its density.
All these (and other) observations lead physicists to reach a consensus that dark matter is present — too many results wouldn’t make sense without it. This leaves researchers with the challenge of finding out what it’s made of.
Searching for new particles
Currently, the leading explanation is that dark matter is some kind of unusual particle, beyond the Standard Model of Particle Physics. This describes all that physicists currently know about the particles and forces that make up the universe.
The Standard Model describes three fundamental forces: electromagnetic, weak and strong nuclear forces. We know that dark matter particles can’t interact with the electromagnetic force (because they’re dark) and that they probably don’t interact with the strong nuclear force (because they don’t seem to bind together into clumps).
This means the only way dark matter could interact with ordinary matter is through the weak nuclear force. Scientists have been trying to build detectors to observe this since the 1980s.
Today, STFC’s Particle Physics Department helps build various experiments which investigate types of particles that are thought to be possible candidates for dark matter. New suggestions are made regularly, but one promising category is known as Weakly Interacting Massive Particles (WIMPs).
WIMPing out
Dark matter detectors are built deep underground to shield them from cosmic radiation, energetic particles from space. They’re also built out of materials that release as little radiation as possible, as both these sources of energy would interfere with the experiments.
Experiments designed to detect WIMPs normally include a tank of cooled liquid. One such experiment, the LUX-ZEPLIN detector, uses ten tonnes of very cold (−100∘C) liquid xenon to look for dark matter candidates. Located a mile underground at the Sanford Underground Research Facility (SURF) in South Dakota, it looks for signals of interaction between a dark matter particle and a xenon atom.
If a dark matter particle hit one of the xenon atoms, it would cause the atom’s nucleus to move, known as a nuclear recoil, releasing photons, or packets of light, and electrons.
Detecting both the photons and electrons would allow you to work out where in the detector the interaction happened and would give you more information about the possible dark matter particle detection.
Stuck in the MIGDAL with you
Other experiments, such as the Migdal In Galactic Dark mAtter expLoration (MIGDAL) experiment built with researchers from the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory, try to improve how detectors such as LUX-ZEPLIN look for dark matter. MIGDAL hunts for the Migdal effect, which was first described by Arkady Migdal in 1939.
He proposed that if a neutral particle, which has no electric charge, hits a nucleus fast enough, it could cause the nucleus to move without bringing all its electrons along with it. This could lead to the moving atom emitting an electron.
It is thought that the Migdal effect may occur within dark matter detectors. Observing the process and improving our understanding of it would increase our detectors sensitivity to low mass WIMPs.
Some proposed dark matter particles are light enough to be produced by collisions at the Large Hadron Collider (LHC), at CERN. If they were created, they wouldn’t be picked up by the detectors, but would carry away energy and momentum — meaning scientists could tell they were there by the missing energy and momentum after a collision.
Shining a light on dark matter
Dark matter is still yet to be discovered, but the many experiments carried out to find it have ruled out a large number of possible dark matter theories.
Collaborations between particle and cosmological researchers are constantly developing our understanding of the universe’s longstanding mystery, with each new discovery narrowing our search for new and exotic particles. Despite the long path to discovery, there could still be a bright future for dark matter.
