No one knows how big the Universe is, but there are certainly billions of galaxies out there, many containing a trillion stars. That’s a whole lot of stuff, without even taking in all the gas, dust, planets and assorted interesting debris of space. Yet astronomers tell us that there’s around five times more matter out there that we simply can’t detect.
This missing stuff is given the misleading name ‘dark matter’, as the first person to theorise its existence, Swiss astronomer Fritz Zwicky, called it ‘dunkle Materie’ in German. Oddly, one thing dark matter isn’t is dark. A dark coloured object absorbs most of the light hitting it –but dark matter appears to be totally transparent. I say ‘appears’, as we have found any dark matter: its existence is inferred from the behaviour of ordinary matter under its gravitational influence.
One effect of dark matter is to stop galaxies and clusters of galaxies flying apart. Rotate something fast enough for the centrifugal force to be greater than the forces holding it together, and it flies apart. You can see this happening on many an amateur potter’s wheel. Galaxies too shouldn’t be able to stay together at the speed they rotate. It’s as if there is more matter we can’t see, holding them together. Similarly, we know from Einstein’s general theory of relativity that galaxies warp space sufficiently to act as lenses, bending the light that goes past them – and the effect is greater than is accounted for by the visible material.
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Although Zwicky first proposed dark matter’s existence back in the 1930s, it wasn’t until forty years later when American astronomer Vera Rubin published data on galactic rotation that the idea was taken seriously. If something solid like a CD rotates, the parts of the CD nearer the edge travel faster than those near the middle – they have to, because they’ve further to go in the same time.
But a galaxy is only loosely connected by gravity and the expectation was that moving out from the centre, rotation speed would shoot up to a maximum, then tail off. Rubin, working with Kent Ford, showed that in galaxies she observed, stars near the edge moved at similar speeds to those near the middle. The most obvious reason for this was if a lot of matter was distributed spherically around the outside the galaxy in what is known as a halo.
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It was first thought that the dark matter would be so-called MACHOs (Massive Compact Halo Objects) – ordinary matter not bright enough to see. But there is no good evidence for this, and ordinary matter would not be expected to form halos.
Whatever dark matter was, it was likely to be influenced by gravity but not the electromagnetic force, meaning it would be unaffected by light or the usual matter-to-matter interaction that enables us, for example, to sit on a chair. This better matched a rival concept to MACHOs, particles called WIMPs (Weakly Interacting Massive Particles). But what exactly could dark matter be?
One candidate had already been discovered – the neutrino, a particle emitted in vast quantities by nuclear reactions. Neutrinos hardly ever interact with matter – tens of trillions from the Sun pass through you every second without any effect. Neutrinos only have a tiny mass, but enough of them could produce the effects of dark matter.
However, neutrinos were discounted as they move too quickly to be easily captured by the weak force of gravity. Observations of the early structure of the Universe from the cosmic microwave background radiation confirm that the dark matter that appears to have been involved in the formation of galaxies was not travelling at neutrino-like speeds.
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Although having mass, neutrinos are not ideal as WIMPs, which theoreticians believe would have significantly higher masses. This suggested a particle outside our standard model of particle physics. One potential candidate emerged from supersymmetry. The particles in the standard model particles break down into two categories – fermions, which are effectively matter particles such as quarks and electrons, and bosons, which are force carriers, such as the photon. Many variants of string theory, one of the proposals to combine general relativity and quantum theory, suggest that each particle has a ‘supersymmetric’ particle of the opposite type.
The lightest class of supersymmetric particle, neutralinos, have been put forward as possible candidates for dark matter. Unfortunately, such particles should be light enough to be produced by the Large Hadron Collider at CERN, yet not a single supersymmetric particle has ever been detected, throwing the whole concept of supersymmetry into doubt. Nor is there any evidence for such particles in the wild from experiments designed to detect dark matter.
With supersymmetric options looking unlikely, another contender is the axion. This hypothetical particle sounds like a washing product for a good reason – it was named after a dishwasher detergent. It’s a particle that was dreamed up to explain an oddity in quantum physics. The axion would have extremely low mass (less than a neutrino), but like the neutrino it would be expected to move too quickly to act as dark matter – and no such particle has ever been detected.
Some physicists, notably Lisa Randall, suggest that by looking for a dark matter particle we are being ‘dark matter chauvinists.’ Randall points out that our standard model includes 17 types of particle – why, then, should dark matter be made up of a single type of particle? We could envisage an unseen parallel dark matter Universe, in which ‘dark light’ shines from dark suns onto dark planets occupied by dark beings. In reality, this is unlikely, as dark matter seems not to interact with itself other than gravitationally, but it’s fun speculation.
As we continue to fail to find any direct evidence of dark matter, another option is gaining ground – that it doesn’t exist at all. There are several theories collected under the banner of ‘modified gravity’. The idea is that, while Einstein’s general theory of relativity is very effective, it needs modifications to deal with large collections of matter such as galaxies. The result would be the observed behaviour without requiring extra matter.
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Modified gravity is attractively simple, but many scientists reject its basic form, MOND (Modified Newtonian Gravity) because there are clear examples of galactic structures, including the Bullet Cluster, which are difficult to explain using a modified gravity theory.
However, equally, dark matter fails to explain the way that many galaxies rotate. Both theories would need some tweaking to deal with every example, and variants of modified gravity are looking quite promising. Alternatively, a hybrid approach where dark matter is a superfluid that provides different behaviour on the scale of individual stars and galaxies has been suggested – though this approach needs significantly more detail to be robust.
It’s also important to consider one last possibility – that there was never a need for dark matter in the first place. Mathematician Donald Saari suggests that the models used to predict the rotational behaviour of galaxies are wrong. A galaxy involves gravitational interactions between billions of stars. Saari suggests that the calculations predicting dark matter involved too much approximation and don’t properly match reality, meaning that no additional matter may be required.
The search continues. Next-generation detectors are being constructed, with several going online in the early 2020s. However, despite a vast amount of effort the search has so far produced nothing concrete. As Vera Rubin ruefully pointed out in 2001, she had predicted back in 1980 that dark matter particles would be directly observed within ten years, yet they still hadn’t been seen. Again, in 2000, Astronomer Royal Martin Rees made a similar ten-year prediction. His timescale is also long past.
We can’t be certain if dark matter will ever be discovered. But this challenging mystery is one of the reasons that science is so fascinating. There is still much to discover – and the cause of the dark matter effect is high on the list.
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