At an age of 380,000 years, the temperature of the universe had fallen to 3000 k, and electrons could then combine with protons to produce hydrogen gas instead of roaming freely through space. what major transition occurred as a consequence of this change in the universe at this time?

The major transition occurred is that the universe became transparent to light for the first time. This is the consequence that might happen because of the changes in the universe at this time. The reaction between the elements that are present in the space can affect the present universe.