Galaxies are thought to form through a process called hierarchical galaxy formation, which involves the interplay of gravity, dark matter, and ordinary matter (such as gas and dust). The formation of galaxies is a complex and dynamic process that occurs over cosmic time scales.
Here is a simplified overview of how galaxies are believed to form:
1. Early Universe Conditions: In the early universe, slight density fluctuations in the distribution of matter occurred due to quantum fluctuations during cosmic inflation. These density fluctuations served as the seeds for the formation of structures in the universe.
2. Dark Matter Influence: Dark matter, which doesn't interact with light and other forms of electromagnetic radiation, played a crucial role. It began to collapse under the influence of gravity, forming large structures known as dark matter halos.
3. Gas Collapse: Ordinary matter, mainly hydrogen gas, was drawn into these dark matter halos by gravity. As the gas contracted, it formed rotating disks or spheroidal structures.
4. Star Formation: Within these gas-rich regions, stars began to form. The first generation of massive stars synthesized heavier elements through nuclear fusion.
5. Galaxy Mergers: Over time, galaxies interacted and merged due to gravitational forces. Mergers played a significant role in shaping the morphology and properties of galaxies. Large galaxies, like the Milky Way, have likely formed through the merger of smaller galaxies.
6. Feedback Mechanisms: The process of star formation is associated with feedback mechanisms, such as supernova explosions and active galactic nuclei. These mechanisms can influence the surrounding gas and regulate further star formation.
7. Evolution: Galaxies continue to evolve over billions of years, with ongoing processes of star formation, stellar evolution, and interactions between galaxies shaping their structures and properties.
Understanding the detailed mechanisms of galaxy formation and evolution is an active area of research in astrophysics, and observational data from telescopes, simulations, and theoretical models contribute to our understanding of these cosmic processes.