Astronomers are still arguing about the precise sequence of events during the Milky Way’s birth, but every-one agrees that the story began with dark matter. The stuff is everywhere, even though it is invisible and no one yet knows what it is. It outweighs ordinary matter — stars, gas and everything else made of atoms — by a factor of about five, and yet can be detected only through its gravitational pull on visible stars and galaxies. Astronomers have known since the 1970s that the Milky Way, like every other galaxy, is wrapped in a vast cocoon of dark matter; without it, the gravity generated by ordinary matter would be nowhere near enough to hold the Galaxy together.
Galaxy formation: The new Milky Way : Nature News & Comment.