If we could don dark matter glasses and look at the universe around us, we might see thousands of miniature galaxies swarming about the luminous spirals that make up the Milky Way and Andromeda.
We can't – but we have the next best thing. A technique known as gravitational lensing has allowed one of these dark dwarfs to be glimpsed, suggesting the Milky Way isn't as lonely as it looks to us Earthlings.
Astronomers think that galaxies usually grow by devouring smaller nearby clusters of stars called dwarf galaxies, no bigger than 100 million times the mass of the sun. According to this theory, the Milky Way and all other full-size galaxies should keep company with thousands of dwarfs. However, only 30 such companions have been spotted in our neighbourhood.
Where are all the missing minis hiding? One explanation is that they're mostly made of dark matter, the mysterious, aloof substance thought to make up 83 per cent of the mass in the universe but which is reluctant to interact with regular matter.
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