According to current observations, about 5 to 10 percent of all large galaxies in the nearby universe contain AGNs or QSOs. The brighter the QSO, the rarer it is. Only a small fraction of QSOs, for example, are as luminous as 3C 273. The farther back you go in the history of the universe, however, the higher the incidence of QSOs becomes. This is an important piece of evidence that the universe is aging and evolving over time.