This Hubble Space Telescope composite image superimposes a blue map of the dark matter distribution in the galaxy cluster Cl 0024+17 on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence to date for the existence of dark matter, an unknown substance that pervades the universe. (NASA, ESA, M.J. Jee and H. Ford, Johns Hopkins University)
Look up to the sky on a particularly clear night and you'll see planets, stars and galaxies. Peer deeper and you'll see clouds of dust and gas and galaxy clusters and superclusters.
Counting all of the stars in the universe is a bit like counting grains of sand on the Earth in that it involves a great deal of guesswork and assumptions, but the European Space Agency said there could be roughly 10 to the power of 24 — or one million billion billion — stars.
And that's not including the other stellar material we can see: clouds of dust and gas, black holes and quasars, for example.
Yet astronomers generally agree these stars and other visible matter account for less than three per cent of the matter in the universe.
Where is the rest of it hiding? In two places. One is a mysterious substance or group of substances called dark matter, which makes up about 23 per cent of the stuff of the universe. The rest of the universe is made up of the even more mysterious dark energy, which is responsible for the expansion of the universe.
Here we try to explain what scientists think dark matter and dark energy are, why scientists think they are there, and what it means to our understanding of the universe.
This 3-D map of the distribution of dark matter in a patch of the universe, going from recent times, on the left, to about 6.5 billion light years ago, on the right. (NASA)
What is dark matter?
A good question, and possibly the hardest to answer. Dark matter is matter that doesn't absorb or emit light and is thus invisible to direct observation. In other words, we can't see it. But astronomers can infer its presence because it does interact with other matter gravitationally, and those results can sometimes be observed.
Why do we think it's there?
The largest piece of evidence of dark matter's existence is in the orbital motion of stars and galaxies. Planets, stars and galaxies orbit larger objects because of the force of gravity: the larger the mass of the object, the more gravity it exerts on nearby objects. But gravity decreases as one moves farther from the mass exerting the force, which is why planets closer to our sun, like Mercury, orbit at faster speeds than planets like Neptune on the outer edge of the solar system.
If visible matter was the only matter at play, the same should be true of stars in orbit around the more massive galactic centres. But that's not the case: stars on the outer rim of galaxies have been observed traveling at the same speed as those closer to the centre, and likewise smaller galaxies have been observed orbiting larger ones much faster than the mass of the larger galaxies would suggest.
In the 1930s, Swiss-American astronomer Fritz Zwicky was the first to propose the results could be because of some unseen matter, which he called "dark matter." Subsequent observations supported the theory, and today most astronomers are now in agreement that most of the galaxies in the universe are surrounded by halos of dark matter, adding mass that has a direct impact on the movement of the visible universe.
There are other theories to explain the motion of stars and galaxies, said Marla Geha, a professor at the NRC Herzberg Institute of Astrophysics in Victoria. But many of these involve modifying the way in which the laws of gravity work, a solution that has other, undesirable ramifications on our understanding of the universe, she said. In other words, changing gravity may cause more problems than it fixes.
If we can't see it and we don't know what it is, why do scientists care?
Dark matter is important to astronomers in part because scientists love a good mystery. But more importantly, anything that makes up such a sizeable chunk of the universe is bound to have an impact on our understanding of everything from the universe's early origins to the way stars and galaxies interact today.
"It's a huge part of the universe, and we know virtually nothing about it," said Michael Kesden, an astronomer with the University of Toronto. "As a scientist, the less we know about something, the more we want to know."
Dark matter is also important not just in the movement of galaxies, but also the origins and shape of the universe: it is often described as the invisible scaffolding of the universe.
Theories of dark matter suggest it formed the architecture in which hot gas from the Big Bang collected — a kind of gravitational well — to form the first stars and galaxies, said Kesden. These stars and galaxies then drew closer to each other, forming the larger and larger cosmic structures we see in the night sky today.
Change the nature of dark matter and you change our understanding of the universe, said Kesden.
"It's absolutely fundamental to our understanding of the large scale structure of the universe," he said.
Are there any dark matter candidates?
Most particles interact in some way with light, or electromagnetically. This includes the protons and neutrons — collectively known as baryons — that form the bulk of the atoms in your body, the chair you sit on and the computer you are viewing.
Some dark matter could be "baryonic" in nature, but it would have to be too difficult for our telescopes to detect — things like small black holes, low-mass stars and Jupiter-sized planets. But most astronomers agree these objects wouldn't be enough to make up the difference in the missing mass.
Another group of more promising candidates are called Weakly-Interacting Massive Particles, or WIMPS. These theoretical particles get their name because they only interact with two of the fundamental forces of nature: gravity and the weak nuclear force.
Because they ignore electromagnetism, WIMPS would be invisible to light or other waves of radiation, and because they ignore the strong nuclear force they would pass through atoms without detection.
It makes them hard to find, but particle physicists have come up with hundreds of candidates based on an array of theories. But none has been detected.
OK, what about dark energy?
Dark energy is even less understood. Its existence was first inferred when astronomers looked at far off supernovas and how fast they were traveling away from the Earth, which they hoped would tell them about the rate at which the universe was expanding. Astronomers expected to find that the universe expansion is slowing. What they found instead was that it is speeding up.
This remains a baffling mystery, and astronomers and cosmologists — astronomers that concern themselves with the history of the universe — have invoked the term "dark energy" to describe the cause of this expansion.
Some astronomers have used Albert Einstein's "cosmological constant" as a means of explaining dark energy. Einstein originally coined the term to describe an energy or force needed to counter the effect of gravity and keep the universe static in size, neither expanding nor contracting. He abandoned the theory, however, after astronomer Edwin Hubble found evidence in 1929 that the universe was expanding.
There is no doubting dark energy's impact. About seven billion years ago, the universe had expanded far enough in size that dark energy overtook dark matter as the dominant substance in the universe, and the expansion has accelerated since.On Dec. 16, 2008, NASA scientists revealed another piece of evidence pointing to dark energy as a force against gravity. Using the Chandra X-ray Observatory to measure the mass of distant galaxy clusters, astronomers discovered the galaxies have either grown marginally or not at all in the last five billion years.
So it appears that while dark energy is pushing galaxies and galaxy clusters further away from each other, it is also stunting the growth of those same galaxies and clusters of galaxies.
Is all this searching getting us anywhere?
This is actually an exciting time in dark matter research. In the last year alone, scientists have produced a 3D map of dark matter in the universe and found evidence of a halo of the substance by observing how its gravitational pull had subtly altered the light from distant stars.
And the search for dark matter has also had an indirect impact on how scientists collaborate, said Kesden.
As astronomers puzzled over what dark matter could be, they found they needed particle physicists to understand some of the more exotic theoretical particles like many WIMP candidates.
Similarly, particle physicists found the early universe to be a fertile ground for studying high-energy particle collisions and began to pay closer attention to astronomical research.
"Forty years ago there was almost no intersection between astronomers and particle physicists. We were looking at the stars and they were colliding protons in particle accelerators," Kesden said. "But since then, we've really grown together."
