What is Dark Matter?

In the past few decades, cosmologists have discovered that 'regular' matter – the stuff we can see and that makes up stars, planets, rocks, gas clouds and dust – only accounts for a small fraction of the total mass in our Universe. Scientists call this 'regular' matter baryonic matter, so called because it is made up of particles called baryons. Dark matter is the name we give to matter we can not observe directly, and that appears to be made up of something other than baryons.

The first evidence for the existence of dark matter emerged in the 1930s when an astronomer called Fritz Zwicky observed the Coma cluster – a massive cluster of over 1000 galaxies, all gravitationally bound to one another. Zwicky looked at how many galaxies were visible in the Coma cluster and made an estimate of the total amount of matter in the cluster based on the mass of an average galaxy. He also measured the velocities of some of the galaxies in the cluster, and deduced that many of them were moving very fast – so fast, in fact, that they should have been able to escape the gravitational pull of the other galaxies in the cluster and escape into deep space. This implied that the cluster was much more massive than Zwicky had calculated; the most natural way to account for this was to assume that much of the cluster's mass was invisible.

Since Zwicky's early observations, similar data from other galaxy clusters has yielded the same result – we consistently see that clusters of galaxies appear to have masses tens of times larger than their luminous matter content can account for. Further evidence for dark matter has since been discovered in individual galaxies themselves. In the 1970s, astronomers noticed that the stars in the outer parts of several nearby galaxies were orbiting their galactic centres faster than expected, and were apparently moving fast enough to escape their host galaxies. This again implies that the mass of the galaxy is much higher than can be accounted for by the visible stars and gas alone. From this, astronomers concluded that dark matter appears to surround galaxies in a halo, extending far beyond the edges of the visible galaxies themselves, as well as existing in the space between galaxies in clusters.

The most interesting thing about dark matter is not simply that we can't see it, it's that we know dark matter is not made of the same stuff as normal baryonic matter. This is actually why we can't see it – baryons interact with each other through gravity, nuclear forces and the electrostatic force. These interactions are what allow baryonic matter (such as stars) to emit light, and what prevent you from putting your hand through a table – the particles of your hand are electrostatically repelled from the particles in the table. Dark matter, however, only interacts through gravity. This is why we see its effects on the motions of galaxies and stars, but why we can't see it directly; it does not emit or absorb light. Dark matter particles can also pass through regular matter almost completely undetected since they don't interact electrostatically, meaning we can't touch it or sense it in any direct way.

One of the reasons astronomers believe dark matter is non-baryonic in nature is because it is possible to calculate how much baryonic matter there actually is in the Universe. By measuring the ratio of hydrogen, the most common element in the Universe, and its heavier isotope deuterium, astronomers have been able to work out how much baryonic matter there must be. This is because deuterium is very difficult to produce, and almost all the deuterium in existence today was formed in the Big Bang. The exact amount of baryonic matter created influenced the hydrogen/deuterium ratio, and as a result we now know that only 4% of the mass-energy content of the Universe is in the form of baryonic matter. We know from other observations that all the matter in the Universe makes up around 23% of the mass-energy content, so the discrepancy is due to the existence of non-baryonic dark matter. This nicely fits in with our observations of galaxies and clusters that show that most of the matter (approximately 5/6th) is in a form we can not see.

So what might dark matter be made of? There are many different hypothetical particles proposed to explain dark matter, but the leading candidates are known as Weakly Interacting Massive Particles, or WIMPs. WIMPs interact with baryonic matter through gravity (as we know dark matter does), and are also expected to interact very slightly through a force known as the weak nulcear force. Simulations predict that dark matter made of WIMPs would produce structures in the Universe that are very similar to what we actually observe. If WIMPs really do interact through the nuclear force, scientists may be able to detect them directly using sensitive underground detectors. There must be many dark matter particles passing through the Earth all the time, and although most pass unimpeded occassionally one may interact with a molecule, producing a tiny flash of light and new decay particles. By looking for these telltale flashes and working out the identity of the decay particles produced in the reaction, it may be possible to deduce the identity of dark matter itself.

Author: Emma Grocutt