The Cold Dark Matter theory posits that right after the Big Bang, gravitational forces influenced the minute variations in density of dark matter, slowly clumping them together into large and larger forms. These clumps eventually became gravity wells that, in turn, pulled together the remaining visible matter into the galaxies we see around us.
As Piero Madau, professor of astronomy and astrophysics at UCSC and coauthor of the study states:
Star formation in real galaxies occurs in a clustered fashion, and to reproduce that out of a cosmological simulation is hard. This is the first simulation that is able to resolve the high-density clouds of gas where star formation occurs, and the result is a Milky Way type of galaxy with a small bulge and a big disk. It shows that the cold dark matter scenario, where dark matter provides the scaffolding for galaxy formation, is able to generate realistic disk-dominated galaxies.
Until now, numerous lower-resolution computer simulations had failed to generate a spiral galaxy similar to our own — slight bulge in the middle with a large, well-formed disk surrounding it — instead creating one with an impossibly large bulge-to-disk ratio. Turns out that this was caused because the low resolution of the models averaged gas densities over too large an area which resulted in low-density stars. In the high resolution study by US Santa Cruz, dubbed Eris, gas densities were more locally concentrated which resulted in a more realistic distribution of stars that only form in the highest density areas.