Final answer:
The thermal radiation spectrum of the universe at 380,000 years old was dominated by cosmic microwave background radiation, evidence of the Big Bang and the beginning of the universe's expansion.
Step-by-step explanation:
When the universe was 380,000 years old, its thermal radiation spectrum consisted mostly of cosmic microwave background (CMB) radiation. The universe had cooled to a point where electrons combined with protons to form neutral hydrogen atoms, making the universe transparent to radiation. The remnant heat from the Big Bang was left as a blackbody radiation spectrum, now observed as CMB. At that time, no stars or galaxies existed, marking the period called the dark ages. Eventually, density fluctuations in dark matter caused the formation of gravitational traps, which led to the creation of galaxies several hundred million years later.
The CMB spectrum is the most perfect blackbody spectrum known, with a temperature of about 2.725 K, as measured by the COBE satellite and other observations. This discovery is compelling evidence of the Big Bang and supports the theory of the expansion of the universe.