What exists in the space between galaxies? Why is the universe expanding so quickly? And what is the mysterious dark matter that binds the galaxies?

Astronomers are living in a golden age of cosmic discovery. Their observatories have revealed the shape, size and age of the universe in unprecedented detail, and their theories are starting to unlock the mysteries of the big bang.

But behind all the headline-grabbing breakthroughs lurks an embarrassing fact of cosmic proportions. Astronomers have no idea what most of the universe is made of, nor what propels its expansion.

It was not always thus. Once upon a time, astronomers believed the universe was filled with bright stars and galaxies, gently expanding under the impetus of the big bang. But this simple picture didn't last long. During the 1930s, an astronomer in California named Fritz Zwicky uncovered evidence that clusters of galaxies were glued together by the gravitational pull of invisible "dark matter". In the 1970s, other astronomers found evidence for huge halos of dark matter surrounding individual galaxies.

Some of it is simply dust and gas lurking within and between the galaxies. But it is now clear that only a small proportion of dark matter can be in the form of conventional matter such as atoms. So what can it be? Over the years, a host of candidates has been put forward, ranging from black holes to ghostly particles known as neutrinos. Many have either fallen foul of observational tests, or remain speculative. But that could be about to change.

Wimps

The current frontrunner is a new kind of particle known as the weakly interacting massive particle or "wimp". Although so far never observed on Earth, wimps are a natural consequence of a phenomenon known as supersymmetry, currently attracting huge interest among physicists.

Put simply, supersymmetry is a fundamental similarity or "symmetry" between the two basic types of particle in our cosmos: those that make up matter, such as electrons, and those affecting matter, such as photons. Supersymmetry predicts that every member of one group must have a corresponding member of the other. Known collectively as "sparticles", they interact only weakly with conventional matter while being relatively massive - hence their moniker of wimps.

Physicists hope to find convincing evidence for supersymmetry in experiments at the Large Hadron Collider (LHC), a colossal particle accelerator about to start operation near Geneva. They might even detect wimps directly, allowing them to see if they really are the answer to the dark matter mystery. If so, physicists will win the eternal gratitude of their star-gazing colleagues. But astronomers still have another huge mystery to solve, this time one that embarrasses physicists as well: dark energy.

Ever since the time of Newton, it has been a scientific article of faith that the cosmos is ruled by gravity. That faith has been dealt a heavy blow by the discovery a decade ago that the universe is not just expanding, but expanding ever more quickly. Observations of distant galaxies have revealed that they are racing from each other at an accelerating rate, seemingly in the grip of some anti-gravitational force that appears literally out of nowhere. Dubbed "dark energy", it appears to have wrested control of the universe about 5bn years ago. Yet its cause and its implications are largely unknown.

The current best bet is that dark energy is a manifestation of the so-called quantum vacuum, one of the most extraordinary implications of quantum theory, the laws of the subatomic world. In the 1920s, the German theorist Werner Heisenberg unveiled his celebrated uncertainty principle, which puts strict limits on what we can know about particles. In particular, it is impossible to know the exact energy content of a region of space over a given period of time. As such, there is no such thing as a perfect vacuum: even a region of apparently empty space is seething with energy. This is not some idle speculation: the reality of quantum vacuum energy has been confirmed in laboratory experiments. These show that we are immersed in a sea of particles and anti-particles, randomly appearing literally out of nowhere before vanishing again, their fleeting existence being detectable through their effect on atoms.

But the effect on the cosmos as a whole of vacuum energy in the form of dark energy is harder to explain. Many theorists suspect real progress with the mystery of dark energy will only come with the creation of the much-heralded theory of everything, a single set of equations linking quantum effects to Einstein's theory of gravity. This may give insights into the role of quantum vacuum energy in the early universe, where its anti-gravitational effect may have triggered the big bang. But it may also cast light on the ultimate fate of the universe, now being propelled towards an unknown future by this most enigmatic of cosmic forces.