Read more: 鈥�Instant Expert: Dark matter鈥�
It is an embarrassing time to be a cosmologist. Only a couple of decades ago, we thought we understood the substances that fill the universe. No more. We now know that the atoms making up everything visible in the cosmos 鈥� from galaxies to planets to clouds of interstellar gas and dust 鈥� represent less than about 20 per cent of the total matter out there. The remaining 80 per cent is mysterious 鈥渄ark matter鈥�, invisible to conventional telescopes. But if we can鈥檛 see it, how can we be so sure it鈥檚 there?
Galaxies in a spin
We can鈥檛 weigh the sun or a planet directly. Instead, we determine its mass by measuring how its gravitational pull influences the motion of objects around it.
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In the same way, it should be possible to measure the mass of a galaxy, or even a cluster of galaxies, by observing how fast stars or other objects move around it. In 1933, the Swiss astronomer Fritz Zwicky (pictured, right), working at the California Institute of Technology in Pasadena, applied this principle to the motion of galaxies that make up the Coma cluster, a group of over 1000 galaxies some 300 million light years from us. He found that the individual galaxies were zipping round far too rapidly for their gravity to keep them bound together in a cluster. By rights they should have been flying off in different directions.
Zwicky鈥檚 puzzling results didn鈥檛 get much attention until the late 1960s, when the astronomer at the Carnegie Institution in Washington DC measured the Doppler shift of clouds of hydrogen gas in several distant galaxies. This showed that the speeds at which the clouds were orbiting the centre of their galaxies seemed to require far more mass than could be accounted for by visible material (see diagram).
The discrepancy between the amount of visible matter and the strength of gravity is most pronounced in some of the very smallest galaxies, known as dwarf spheroidals. These objects contain as few as tens or hundreds of thousands of stars, but produce a gravitational attraction equivalent to tens of millions times the mass of our sun. Even our own Milky Way galaxy generates a gravitational pull of an object of roughly 800 billion solar masses, despite containing a total visible mass of only a couple of hundred million suns.
Without dark matter, the very existence of many apparently stable galaxies would defy the laws of physics. The fact that they do exist remains among the most compelling reasons to think that there must be more to the cosmos than meets the eye.
Uneven background
Although we still can鈥檛 see the stuff itself, we see evidence for dark matter everywhere we look, for example in the radiation known as the cosmic microwave background (CMB), which was created in the infancy of the universe.
About 380,000 years after the big bang, the temperature of the universe dropped below about 3000 degrees kelvin, making it possible for the first time for atoms to form (see diagram). The transition from disconnected nuclei and electrons to electrically neutral atoms released a huge amount of energy in the form of light, and the expansion of the universe has since stretched this light to microwave wavelengths. This radiation today fills all of space, a relic of our universe鈥檚 hot youth.
By studying the patterns of slightly hotter and colder patches in the CMB, we have been able to learn a great deal about our universe鈥檚 history and composition. Among other things, these variations in the CMB tell us how matter was distributed throughout space in the early universe. Because dark matter began clumping under the influence of gravity earlier than normal matter did (see 鈥淭he invisible hand鈥�), its influence can be seen in numerous small hot and cold patches, each covering an angle in the sky of 0.25 degrees or so.
The pattern of these spots even allows us to determine how much dark matter must be present. It turns out that for every gram of stuff that we can see in the cosmos there must be 4 or 5 grams that we can鈥檛. That doesn鈥檛 even include another, perhaps even more mysterious, substance whose existence can be inferred from the CMB: dark energy, a force that seems to be causing our universe to expand ever faster. Totting up all the mass and energy in the universe, dark energy trumps normal matter and dark matter combined by a factor of almost 3 to 1.
The invisible hand
Even if dark matter weren鈥檛 needed to prevent galaxies flying apart, suggest that the cosmos would look very different if it didn鈥檛 exist. These simulations track the movement of billions of particles through cosmic time, with the aim of better understanding why the universe has ended up the way it has.
When atoms in a gas of ordinary matter are compressed, they collide more frequently. This interaction tends to push the atoms apart and so hinders gravity from compressing the gas any more. Dark matter particles, on the other hand, interact with each other only feebly and so clump much more readily. Simulations that embody these properties show that as the universe expanded and evolved, the first structures to form would have been clumps, or 鈥渉alos鈥�, of dark matter.
The first dark matter halos to form were probably about as massive as the Earth, but far more diffuse. Over time, they began to merge and became steadily larger. Eventually, some became massive enough to attract large quantities of hydrogen, helium and other conventional matter 鈥� the seeds of the first stars and galaxies.
The agreement between the shapes and sizes of the structures derived in dark-matter simulations and those observed in our universe is striking (see picture). That leaves little doubt that dark matter is not only real, but also that it formed the nurseries in which galaxies such as our own Milky Way formed.
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