Final answer:
As the universe cooled after the Big Bang, protons and electrons formed hydrogen, making the universe transparent to light and leading to the Cosmic Microwave Background. This transparency allowed photons to travel freely and marked the beginning of galaxy formation as dark matter gravitational traps pulled ordinary matter together.
Step-by-step explanation:
The Universe Becomes Neutral and Transparent
After the Big Bang, a crucial phase in the evolution of the universe took place when it had cooled to about 3000 K, 380,000 years post-event. During this time, protons and electrons combined to form hydrogen atoms, creating a major shift in the universe's properties. One of the most important consequences was that the universe became transparent to light, allowing photons to travel freely through space. This marked the decoupling of matter and radiation, and the Cosmic Microwave Background (CMB) radiation emitted at this time provides us with critical information about the early universe.
Following this, the universe went through a period known as 'the dark ages,' during which the first galaxies began to form. This period saw small fluctuations in the density of dark matter forming gravitational traps, pulling ordinary matter into these regions, eventually leading to galaxy formation several hundred million years later. The detection of the CMB and other observations have given astronomers significant insight into the composition of the universe, including the presence of dark matter and dark energy.