Structure from Uniformity

Theoretical investigations indicate that structure on the scales summarized in the adjacent figure formed from a dilute but slightly nonuniform mixture of hydrogen and helium gas -- the primordial elements forged in the big bang. However, the way in which this structure evolved is a difficult problem that is only partially understood.

Rapid Formation of Galaxies

We have seen evidence that recognizable galaxies were already present in the very early Universe, less than a billion years after its birth. (Recall this animation concerning the timescale for galaxy formation.) On the other hand, the discussion of the big bang shows that the Universe must have been remarkably smooth shortly after its formation. Thus, we must understand how the Universe generated galaxies and clusters and superclusters and Great Walls from an almost smooth starting point on such a short timescale.

Seeds and Dark Matter

We believe that the key ingredients are dark matter, which constitutes most of the mass of the Universe, to provide the gravitational attraction to clump matter on the required scales, and initial perturbations ("seeds") around which structure could grow. It is far from clear exactly how this happened, but we have some ideas.