Final answer:
When the Earth cooled to 3,000 K, it was during the early stages of the universe's formation. The universe became transparent to light, but there were no stars or galaxies yet. The formation of galaxies occurred due to small density fluctuations in dark matter.
Step-by-step explanation:
When the Earth cooled to 3,000 K, it was during the early stages of the universe's formation. At this temperature, electrons combined with protons to form hydrogen atoms, causing the universe to become transparent to light. However, during this time, there were no stars or galaxies yet, which led to a period known as the 'dark ages' where the universe was not illuminated. Eventually, small density fluctuations in dark matter led to the formation of gravitational traps that concentrated ordinary matter, resulting in the formation of galaxies.