Final answer:
The early universe was hot and at high density, filled with a mixture of particles and energy. It expanded and cooled over time, leading to the formation of galaxies from regions where matter was concentrated. The correct answer is a.
Step-by-step explanation:
The early universe was a very hot and dense environment. According to scientific models, a nearly uniform, hot "soup" of particles and energy existed at the beginning of time, which included neutrons, protons, electrons, neutrinos, and atomic nuclei like deuterium, helium, and a small amount of lithium.
As the universe expanded, it cooled down, and the hot gas present at that time was vibrating in a way similar to sound waves. These vibrations concentrated matter into high-density peaks and left emptier spaces between them. When the cooling of the universe occurred, the concentrations of matter were "frozen in" leading to the ultimate formation of galaxies within these high-density regions.
To summarize, the early universe was hot and at high density, experiencing fluctuations in density and temperature, with denser regions potentially appearing as colder spots due to their higher mass attracting additional matter. As the universe expanded, the overall temperature decreased, underscoring a dynamic environment that spawned the complex structure of galaxies and clusters we observe today. The correct answer is a.