Let's understand how a small patch of slightly denser gas in the early universe might lead to a galaxy:

1. **Initial State:** Imagine a region of the early universe, say 1 million light-years across, where the gas density is slightly higher than average, perhaps 0.00000000001 kg per cubic meter, compared to 0.000000000009 kg per cubic meter elsewhere.
---2. **Gravity's Pull:** Due to this tiny difference in density, the slightly denser region has a bit more gravitational pull. It starts attracting surrounding gas and dust particles towards its center, like a magnet pulling iron filings.
---3. **Slow Growth:** Over millions of years, more and more matter falls into this denser region. The central part becomes even denser, and its gravity becomes stronger, accelerating the collection of material.
---4. **Star Formation Begins:** As gas clumps together, its pressure and temperature increase. Eventually, parts of these dense gas clouds become hot and dense enough for nuclear fusion to start, forming the first stars.
---5. **Galaxy Emerges:** Billions of stars form within this large cloud of gas and dust. These stars, along with the remaining gas and dust, and invisible dark matter, begin to orbit a common center, eventually taking on a distinct shape like a spiral or an ellipse, forming a galaxy.
---6. **Answer:** This process, driven by gravity acting on initial density fluctuations, is how galaxies like our own Milky Way are believed to have formed and grown over cosmic time.