Since we are meeting in the Isaac Newton Institute, it's fitting to recall that Newton himself envisaged structures forming via ``gravitational instability''. In an often-quoted letter to Richard Bentley, the Master of Trinity College, he wrote:
`If all the matter of the universe were evenly scattered throughout all the heavens, and every particle had an innate gravity towards all the rest, and ... if the matter were evenly dispersed throughout an infinite space, it could never convene into one mass, but some of it would convene into one mass and some into another, so as to make an infinite number of great masses, scattered at great distances from one another throughout all that infinite space. And thus might the sun and fixed stars be formed. ... supposing the matter to be of a lucent nature.'
(It would of course be wishful thinking to interpret his last remark as a premonition of dark matter!)