He concluded that some massive invisible force must also be at work, but scientists have been unable to detect and identify such matter.
In the latest study, the British team used the Very Large Telescope in Acatama, Chile, to gauge the temperature and distribution of dark matter in ten dwarf galaxies around the Milky Way.
"We measured how big a galaxy is and how fast its stars are moving," Gilmore said. "Using Newton's Laws we worked out how much mass there is holding it together."
Each galaxy was found to contain the same amount of dark matter, equivalent to roughly 30 million times the mass of the sun.
"That was the big surprise," Gilmore added. "The galaxies contain very different numbers of stars, so like everybody else we thought they would cover a very wide range of masses, but they don't. There seems to be a minimum mass for a galaxy."
The finding suggests dark matter has a density equivalent to four atoms of hydrogen per cubic centimeter of space, meaning the smallest volume it can be packed into is a cube measuring 1,000 light years along each side.
This in turn implies how fast its particles moveat a speed that doesn't allow dark matter to be compressed any further.
"This property of its minimum speed looks like the first property we've got [for dark matter]," Gilmore added.
The study team's results haven't yet been published and scrutinized by peers. But astrophysicist Bob Nichol from the University of Portsmouth, England, believes the findings represent "a quantum leap in our understanding of dark matter."
The research, Nichol says, "is potentially telling us that you can't squeeze a lot of dark matter into a small space. If you do it will push back, which is where this temperature comes from. It's exceptionally exciting."
Most scientists had assumed previously that dark matter was a denser, relatively cold, sluggish substance.
The new findings suggest dark matter interacts with more than just gravity, Nichol adds.
"There seems to be a pressure to dark matter," he said. "That's telling us there is some interaction either with itself or with the ordinary matter."
Nevertheless, these new clues to the properties of dark matter won't necessarily make it any easier for scientists who are currently trying to record the elusive substance.
"If anything this is going to make it harder for them, because it's easier to find a heavier particle than a lighter-mass particle, as the lighter ones have that much less energy," Gilmore, the study team leader, said.
The best short-term hope, Gilmore says, is the world's largest particle accelerator, the Large Hadron Collider, due to open next year in Geneva, Switzerland.
The eight-billion-U.S.-dollar, 17-mile-long (27-kilometer-long) underground installation aims to smash protons together at the speed of light, recreating conditions that existed a fraction of a second after the Big Bang.
According to Big Bang theory, the universe violently emerged from an enormously dense, hot state and has been expanding ever since. (Read a related National Geographic magazine feature.)
"The stuff we're looking for is the most common form of mass [in the universe], so it must have come straight out of the Big Bang," Gilmore said.
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