Read more: “Instant Expert: Dark matter“
The short answer is that we don’t know what dark matter consists of. It must be invisible, or at least very faint, so it cannot be made of anything that significantly radiates, reflects or absorbs light. That rules out conventional atom-based matter. Other observations provide further clues to its identity.
MACHO or WIMP?
We once thought that dark matter might be made up of large objects such as black holes or exotic types of faint stars – neutron stars or white dwarfs – that are nearly invisible to our telescopes. But observations seem to have ruled out these “massive astrophysical compact halo objects”, or MACHOs.
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The concentrated gravity of a MACHO would deflect passing light on its way to us from distant stars. We do observe such “gravitational lensing” effects, but only often enough for MACHOs to account for at most a few per cent of the mass we do not see. So most cosmologists now think instead that we are submerged in a sea of dark matter – a gas of “weakly interacting massive particles”, or WIMPs – that pervades the entire volume of our galaxy, including our solar system.
Hot or cold?
The only particles we know about that are both stable and do not carry electric charge – and so do not interact with light – are the elusive entities known as neutrinos. Might they be dark matter?
Unfortunately not. Neutrinos are very light and fast-moving, or “hot”, and so resist gravity’s efforts to clump them together. For galaxies and even larger structures to have formed with their observed shapes and sizes, dark matter particles must have been moving slowly, far below the speed of light, over much of the universe’s history. Dark matter must be quite “cold”.
What might this lethargic gas of invisible matter be made of? None of the many types of particles discovered over the past century fits the bill: not electrons, quarks, muons, Z bosons or any other known form of matter. Dark matter must be something completely new. Proposals for dark matter’s identity range from heavy, neutrino-like particles, to ultra-light and cold species of matter known as axions, to truly bizarre possibilities such as particles that are moving through extra dimensions of space.
Dozens of different possibilities have been suggested over the years. To many physicists, however, there is a clear favourite among them: particles predicted by a class of theories that goes by the name of supersymmetry (see “An elegant symmetry”).
An elegant symmetry
Few ideas currently enthral particle physicists more than supersymmetry. The theory is mathematically elegant and could solve some persistent problems – including, perhaps, the nature of dark matter.
In our world, there are two classes of particles: fermions and bosons. Fermions are particles such as electrons, neutrinos and quarks that make up what we normally think of as matter. Bosons are the particles responsible for transmitting the forces of nature. The electromagnetic force, for example, is nothing more than bosons – photons, in this case – shuttling back and forth between electrically charged particles.
Supersymmetry postulates that fermions and bosons cannot exist independently of each other: for each type of fermion, a type of boson with many of the same properties must also exist. The electron, for example, has an as-yet undiscovered bosonic partner called a selectron. Similarly, the photon should have a fermionic analogue known as a photino.
Among the many new particles predicted by supersymmetry is one that is likely to be stable and have all the characteristics required of a viable dark matter candidate. It is the lightest version of a class of particle known as a neutralino. Supersymmetric theories contain at least four neutralinos, which are quantum-mechanical mixtures of the superpartners of the photon, the Z boson that transmits the weak nuclear force and as-yet undiscovered Higgs bosons. Tantalisingly, if neutralinos do exist, the lightest version would probably have been produced in the first seconds after the big bang in quantities similar to what is needed to account for the dark matter in our universe today.
There is, of course, a catch: to date, no one has seen a supersymmetric particle. Physicists generally suspect that the superpartner particles – if they exist – are considerably heavier than their ordinary counterparts, making them very difficult to create or discover in experiments. Huge particle accelerators such as the Large Hadron Collider are on the case (see “In the accelerator”), but until we have hard evidence, the supersymmetry hypothesis will continue to be just that – a hypothesis.
Did we get gravity wrong?
Is dark matter strictly necessary? In 1983, the Israeli physicist Mordehai Milgrom suggested that the higher-than-expected speeds of stars moving around galaxies might be explained another way – if gravity worked differently than predicted by the theories of Newton or Einstein. In particular, he pointed out that the observed galactic rotations could be explained if Newton’s second law of motion – force equals mass times acceleration, or F = ma – were modified to make the force of gravity proportional to the square of the acceleration at very low accelerations.
In recent years, however, Milgrom’s proposal – called MOND, for “modified Newtonian dynamics” – has suffered some serious setbacks. In particular, it has not managed to explain convincingly the dynamics of galaxies within clusters. Observations in 2006 revealed a pair of merging galaxy clusters, known collectively as the Bullet cluster, whose motion indicated that their gravity was not centred on the gas and stars, as would be expected according to MOND.That suggests dark matter has shifted the centre of gravity elsewhere (see picture).
While some cosmologists don’t yet accept that the evidence against MOND is conclusive, most no longer consider it to be a viable alternative to dark matter.
Read more about dark matter in our Instant Expert special
