When we look up at the night sky we can see about 2000 stars with our own eyes. With the aid of a telescope we can see the still more distant galaxies. The time in universe’s history before the first stars or galaxies is referred to as the dark ages.
What happened before stars emerged from the darkness? Even in such times, we still expect for all the building blocks of stars to be in place, and these building blocks are just the atoms of hydrogen. In addition to individual hydrogen atoms zipping around, we also think that the dark ages were a time of a great building up of infrastructure. As near as we can tell, it is not cement and rebar that is used to lay out the cities and build the roads. Instead, there is some kind of matter responsible for the infrastructure building which exists which we cannot see but which responds to gravity. We call this the dark matter, and it appears that the dark matter formed a series of networks which look rather like a multitude of spider webs all connected together and in three dimensions. These networks can act a bit like a highway, because the hydrogen atoms travel along the filaments of this ‘cosmic web’ much like cars move along on a new road. As long as there is someone to steer, cars can stay on a road quite well owing to gravity
which pulls the car towards the center of the Earth. Analogously, the hydrogen atoms stay on the networks of filaments also because the atoms are pulled by gravity, which in this case is produced by the dominant road material (the dark matter). Unlike cars, the atoms do not run out of gas, but keep moving until they reach the centers or 'nodes' of the various webs. It is in these nodes that the atoms stop and build up in sufficient numbers in a small volume enabling a star to be born. Today when we look at galaxies across vast stretches of space, indeed we see a gigantic and remarkably tidy pattern that appears to be embedded into space, and tells us where those natural highways are.