...a Residue of the Primordial Light... This indicates that the light that remained was not new and different from the light that disappeared, but rather that it was like a small part of it. Prior to the act of Tzimtzum, it was not recognizable as something separate in itself, but after the Tzimtzum it could be recognized as such.
We will be able to gain a deeper understanding of this entire matter by considering the relationship between Malchut and the other Sefirot as found in the lower realms from Atzilut downwards. In other words, the relationship between Malchut and the other Sefirot, as found in the worlds of Atzilut and below, will provide an analogy through which we may understand the relationship between the Residue and the departed Primordial Light of Eyn Sof. This is because the Residue is called Malchut of Eyn Sof.