Final answer:
Astronomers find that the number of quasars peaks when the universe was much younger, around 20% of its current age, providing important clues about the early stages of cosmic structure formation.
Step-by-step explanation:
When astronomers count how many quasars there are at different distances from us, they find that their number was greatest when the universe was only 20% of its present age. Due to their incredible luminosity and large redshifts, quasars serve as cosmic lighthouses, illuminating the conditions in the early universe. With the aid of tools such as the Hubble Space Telescope, we've learned that quasars exist in the centers of galaxies and their light reaches us from a time when major cosmic structures were first forming, providing insights into the evolution of quasars and the broader universe.