Cambridge University researchers have creaked open the door to one of the greatest mysteries in science. For the first time they can describe some physical properties of "dark matter", the mysterious substance that outweighs all the stars and galaxies that can be seen in the universe.

Cosmologists know that the stars and planets we can see add up to only 4% of the mass required to keep the universe in its ordered state. The rest is made of a combination of unknown particles called dark matter and a source of energy, which seems to push galaxies apart, called dark energy. Other than knowing that both these things must exist, scientists have been at a loss to describe anything about them.

But by studying the motion of dwarf galaxies orbiting the Milky Way, Gerry Gilmore, the deputy director of the Institute of Astronomy at Cambridge University, calculated that dark matter moved at 5.6 miles a second and that the smallest chunks it could exist in measured 1,000 light years across and had 30m times the mass of the Sun.

"This is the first time we've determined a property of the dark matter robustly in a way that we expect will give us some real clues as to what the real physics of this stuff is," said Professor Gilmore at a briefing in London. He said the universe appeared to be built out of these invisible 1,000 light-year-wide bricks of dark matter.

"There must be some basic property of the dark matter that limits it in that way," he said. "It's the basic unit from which bigger things are made up. Some of these you put stars in and you call it a little galaxy; sometimes you put several of these together and call it a bigger galaxy. But you never get anything smaller."

The biggest surprise is that dark matter is not the cold cosmic sludge that scientists once thought. Prof Gilmore calculated its temperature to be in the tens of thousands of degrees, although this is not normal heat. "Normal hot things glow and you can feel the infrared coming off," he said. "The strange thing about dark matter is that it doesn't give off radiation." This is because dark matter is not made of electrons and protons, the fundamental particles that everything else consists of.

Whatever its mysteries, dark matter has its uses. It is essential in keeping the universe ordered and, without it, the galaxies would quickly fall apart. "The Sun is moving so fast that if it weren't for the dark matter, it would fly straight off out of the Milky Way," said Prof Gilmore. "The reason we are still here is that we're held here by the dark matter."

Different regions of space have different amounts of dark matter. The concentrations can be measured in terms of the equivalent weight of hydrogen, the lightest atom in the universe, per cubic centimetre. Around the Sun the concentration of dark matter is equivalent in weight to a third of an atom of hydrogen per cubic centimetre. According to the new results, the maximum density that dark matter can be packed into is much greater: the weight of four hydrogen atoms per cubic centimetre. While diffuse, it permeates the entire universe and adds up to more than five times the mass of all the stars and galaxies in existence.

The results, which are yet to be published, were obtained by analysing measurements made at the Very Large Telescope, an array of four 8m telescopes on the Paranal mountain in Chile, part of the European Southern Observatory.

The observations took 23 nights of work, the biggest British experiment carried out at Paranal. Prof Gilmore said he was in the final stages of drafting a paper on the results to be submitted to a scientific journal.

The research might also give clues to the relationship between dark matter and dark energy. "Something has fine-tuned the relative amounts of this stuff to make them similar in amount and exactly right to add up to perfection. That can't be chance, there's got to be some connection between the two," said Prof Gilmore.

An additional unexpected result that came out of the dark matter study was the discovery that the Milky Way was bigger than cosmologists had thought. Prof Gilmore said it was the biggest galaxy in the local group, knocking Andromeda, previously thought to be the largest, into second place.

FAQ: Missing mass

What is dark matter?

It was first proposed in 1933 by Swiss cosmologist Fritz Zwicky as a way to explain the missing mass of the universe. Estimates suggest that normal matter - what we see in the universe, including the stars and planets - makes up only 4% of the universe.

How much of it is there?

Dark matter makes up about 23% of the mass of the universe and the remainder is dark energy, another mysterious substance that pushes matter apart.

What do we know about dark matter particles?

Not much. Whatever they are, dark matter particles are transparent to light, and unlike most components of ordinary matter, have no electric charge. Yet they are weighty enough to exert a gravitational pull that prevents the stars in galaxies from flying apart.

Are scientists trying to detect it?

Yes. Experiments are attempting to measure the presence of dark matter using huge, one-tonne crystals kept at low temperatures. The passage of a dark matter particle through the crystal will, very occasionally, lead to a gravitational drag in some of the particles in the crystal. Aside from these indirect measurements on Earth, there is no way of observing the particles yet.