As cosmologists, the main paradigms we work under are known (rather inapproprately) as the big-bang model for the global evolution of the universe and the gravitational instability paragdigm for the formation of objects or structure in the universe.
The big bang model says that the universe began hot and dense but is expanding and cooling.
The gravitational instability models says: we know large masses like the earth attract, for example by the fact that you remain on the surface of the earth rather than flung off of it as it spins, like water off a wet dog. Even a small mass attracts, so that small ripples in the mass density early on in the history of the universe can grow into the galaxies we see today in the night sky.
