Final answer:
Quasars were most numerous when the universe was about 20% of its current age, coinciding with the peak of star formation.
They are rare at low redshifts because the fuel for accretion around supermassive black holes has diminished, and at very high redshifts, galaxies had yet to accumulate enough material to feed quasars.
Step-by-step explanation:
The presence of quasars at different redshifts can be attributed to the changes in the universe over time. Observations indicate that quasars were most numerous when the universe was about 20% of its current age, or during the redshifts between approximately 2 and 3.
This era aligns with the peak of star formation activities across the cosmos, suggesting a possible connection between the fueling of quasars and the abundance of raw material available for star formation.
Quasars are powered by accretion discs around supermassive black holes at the centers of galaxies, and as galaxies evolve, the available fuel for these quasars decreases, which causes the drop-off in their numbers.
In simpler terms, there were more quasars at redshifts 2 to 3 because galaxies were young and had plenty of material to feed these energy-producing regions.
They become less numerous at low redshifts because most supermassive black holes have devoured the accessible matter around them, leading to less active quasars. Similarly, at high redshifts, galaxies were just beginning to form, so not enough materials had accumulated for quasars to become as luminous and noticeable as they would at redshifts 2 to 3.