— The large bright glow in this simulation represents the colossal halo of dark matter surrounding the Milky Way as it was about 3.4 billion years ago. Smaller clumps of dark matter are also visible because they do not merge into larger structures. Curiously, many of these small clumps do not seed dwarf galaxies. The image shows a region of space about 2.6 million light years across (Illustration: J Diemand/M Kuhlen/P Madau/UCSC)
About 12 billion years ago, the universe had not expanded as much as it has today, so more matter was squeezed into a 2.6 million-year-wide region of space. Some of that matter expanded out of the region, but some collapsed back to form the Milky Way's large halo (see upper image) (Illustration: J Diemand/M Kuhlen/P Madau/UCSC)
Our galaxy could be surrounded by a vast swarm of invisible companions. These giant clouds of dark matter the failed seeds of galaxy formation may be detected with a telescope to be launched next year.
Dark matter makes up about 82% of all the matter in the universe, although nobody knows what it actually is. Small clouds of the stuff are thought to have coalesced after the big bang, and then gradually merged together. When enough dark matter is gathered into a huge "halo", it attracts ordinary gas to form stars, and so becomes a galaxy.
Now, scientists led by Jürg Diemand of the University of California in Santa Cruz, US, have modelled this process in more detail than ever before, following the fate of more than 200 million cloudlets as they come together into a halo about the same size as that of our Milky Way.
Such a detailed simulation requires a lot of computing power. "It's at the limit of what current supercomputers can do," Diemand told New Scientist.
The result is a fascinating animation of galactic birth (4.6 MB MPEG version).
It shows there should be at least 10,000 separate "subhaloes" of dark matter within the overall galactic halo, each at least a few thousand light years across.
And a fair number of these galactic seeds should have germinated. About 120 clumps are large enough to have attracted some gas of their own to become dwarf galaxies yet astronomers have seen only 15 dwarf companions of the Milky Way. What happened to the rest?
A relatively radical solution is that dark matter has some property that prevents it from forming dense clumps. For example, it might be unexpectedly hot, and therefore hard to compress. "But at the moment, the consensus is you don't need to go to these extremes," says Diemand.
There are other explanations. It may be that most of the subhaloes were sterilised by ultraviolet light from the earliest stars. That heated up intergalactic gas, making it more difficult to capture. And perhaps supernova explosions blasted gas out of many of the nascent dwarf galaxies, ending their brief lives.
Spotting these dark clouds will be a challenge. If astronomers are lucky, then dark matter particles behave in such a way that they annihilate each other when they collide.
In that case, they would emit gamma rays, and the gamma-ray glow of our dark matter companions might be detectable with NASA's Gamma-ray Large Area Space Telescope, due for launch in 2007.
Otherwise, the clumps might show up as flaws in a gravitational lens. The dark matter haloes of fairly distant galaxies can sometimes focus the light from objects behind them.
The haloes bend light by the force of gravity rather than the refraction of an ordinary lens, often magnifying and multiplying the images of very distant sources. If the haloes are clumpy, that should slightly alter the images produced.