Final answer:
Astronomers find that quasars were most numerous when the universe was about 20% of its current age, and through observations, especially with the Hubble Space Telescope, they study the universe's early formative years.
Step-by-step explanation:
When astronomers count the number of quasars at different distances from us, they find that the population of quasars was at its peak when the universe was only about 20% of its current age. Quasars are extremely luminous objects found at the centers of galaxies, emitting light with very high redshifts, which correspond to high velocities and great distances from Earth.
By observing quasars, we can study the universe at different points in its history, especially during its formative years, billions of years ago. The Hubble Space Telescope has been instrumental in observing these distant objects, revealing that both spiral and elliptical galaxies can harbor quasars, and helping us understand the massive black holes believed to power them.