Final answer:
Galaxies were formed early in the universe's history with a key common assumption being the top-down scenario of galaxy formation, which suggests a rapid collapse of gas and dark matter into stars. Galaxy evolution is influenced by mergers and the environment, shaping their final structure.
Step-by-step explanation:
The question about how galaxies were first formed involves understanding a common assumption in the astrophysical models used to describe galaxy formation. One of the main models for galaxy formation, which is a top-down scenario, presumes that massive elliptical galaxies were formed from a single, rapid collapse of gas and dark matter, where almost all the gas was quickly transformed into stars. Once these galaxies were formed, they underwent only slow changes as the stars within them evolved.
Furthermore, galaxies are observed to have formed very early in the evolution of the universe and have undergone significant changes and growth over time. Galaxy mergers, as well as the formation of quasars within the early elliptical galaxies, play an integral role in the ongoing development and evolution of galaxies. These processes have been vital in shaping the characteristics of galaxies, such as whether they become spiral or elliptical, in combination with the influence of their environment.