That's not a stupid question at all! In fact, it's an intriguing aspect of the universe that scientists have been studying. The reason the universe consists of billions of smaller galaxies rather than one extremely large galaxy can be attributed to several factors:
Cosmological Principle: The Cosmological Principle is a fundamental assumption in cosmology that states that, on large scales, the universe appears the same no matter where you observe it from. This principle suggests that the distribution of matter in the universe should be homogeneous and isotropic. In other words, the universe should look roughly the same in all directions. If the universe consisted of one massive galaxy, it would violate this principle.
Gravitational Collapse: Galaxies form through a process called gravitational collapse. Initially, the universe had tiny fluctuations in density due to quantum fluctuations during the early stages of its formation. Over time, under the influence of gravity, these density fluctuations led to the formation of structures, including galaxies. These structures formed independently and grew through the accumulation of matter in their respective regions. Thus, the universe naturally gave rise to multiple galaxies rather than a single massive one.
Hierarchical Structure Formation: According to the currently accepted cosmological model, known as the Lambda-Cold Dark Matter (ΛCDM) model, large-scale structures in the universe form in a hierarchical manner. Smaller structures, such as dwarf galaxies, form first and then merge to create larger structures, such as spiral or elliptical galaxies. These galaxies, in turn, can further merge to form galaxy clusters. This hierarchical growth contributes to the formation of a diverse range of galaxies rather than a single dominant galaxy.
Dark Matter: The presence of dark matter, a mysterious form of matter that does not interact with light or other electromagnetic radiation, plays a significant role in galaxy formation. Dark matter provides the gravitational framework that allows ordinary matter (such as gas and dust) to come together and form galaxies. The distribution and properties of dark matter influence the formation of galaxies on various scales, contributing to the diversity observed in the universe.
It's important to note that our understanding of galaxy formation and the large-scale structure of the universe is still an active area of research. Scientists continue to explore and refine models to better explain the origins of galaxies and the overall structure of the universe.