Quasars also provide some clues as to the end of the Big Bang 's reionization. The oldest known quasars (z = 6) display a Gunn–Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the Lyman-alpha forest ; this indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.