Dark matter was first postulated in 1933 to explain the amount of gravity necessary to hold galaxies together, and constitutes about 85% of the matter in the cosmos. Because dark matter neither reflects nor emits light, it cannot actually be seen. Now, researchers at the California Institute of Technology (Caltech) in Pasadena, with collaborators from five other countries, offer the first glimpses of how this mysterious matter is arranged. It seems to crisscross the Universe like a massive scaffold, holding ordinary matter, such as planets and stars, in place.

The best method to locate dark matter is to study the way its gravity bends light from objects beyond it, a technique known as gravitational lensing. “We looked at ordinary galaxies behind the dark matter,” explains Richard Massey, lead author of the study on page 286. “Their light has to pass through dark matter to be detected by our telescopes.” Because during this process the light bends, the shapes of the galaxies are slightly distorted.

In a collaborative effort headed by Nick Scoville at Caltech, Massey and his colleagues collected images of half a million distant galaxies using the space-based Hubble Space Telescope (HST). The dense packing of galaxies provides abundant information and thus better resolution to see dark matter's structre. But only from the HST's perch above Earth's atmosphere is it possible to see such an enormous number of these distant, faint objects.

Before Massey and his colleagues could use the data they had collected, they had to tackle some unforeseen problems. As it orbits Earth, the HST travels in and out of Earth's shadow, contracting and expanding as it cools down and heats up again. “The Hubble Telescope effectively breathes as it orbits Earth,” says Massey. The breathing only changes the shape of the ∼13-metre-long instrument by a few micrometres, but even this tiny change puts images out of focus. In fact, the shapes of galaxies are altered by an amount similar to that of the distortion caused by the dark matter.

To remove this artefact from the data, the scientists turned the HST on stars within our own Galaxy. As the precise shapes of the Milky Way's stars are known, the team was able to use them to calibrate the images of distant galaxies.

But they encountered other problems. For example, the electronic detector on the HST started to malfunction after years of being beaten by space particles. As a result, the scientists had to compensate for an increasingly blurred signal as data collection progressed.

After resolving each problem, Massey would return to his dark-matter map and refine it “until we got something that made sense”. The picture that emerged, spanning an area equivalent in size to nine full moons, was one of long filaments of dark matter that meet at large structures marking the locations of galaxies. “On the whole, it is consistent with what we expected,” says Massey. By and large, the map reinforces the theory that dark matter and ordinary matter coalesce at the same places because gravity pulls them together. Concentrations of dark matter should attract visible matter and, as a result, assist in the formation of stars and galaxies.

Gravitational lensing as a technique was first implemented just seven years ago, notes Massey, who wrote his PhD on the topic at the University of Cambridge, UK, before joining the Hubble project in 2004. “And now, at the start of 2007, we have mapped out the dark matter in the Universe,” he says. “It's incredibly fast progress.”