Indeed, as long as the Unlimited held sway, everything was completely perfect. As soon as the Unlimited departed, Malchut was revealed as a “container”, but initially it was not revealed as being filled with all its worlds, but rather as an empty space. Nevertheless, it was not completely hollow, for the root of the worlds that were destined to emerge was present in the form of the Residue, except that the Residue was like air, in the sense that the worlds were not discernible.
Accordingly, even though its container was full (i.e. of air) it is called “hollow” in relation to Eyn Sof, blessed be He. For initially, prior to the Tzimtzum, there was no need for any further perfection, and even after the Tzimtzum, the encompassing Eyn Sof lacks no perfection whatever. That which is still in need of perfection is called “hollow” – i.e. lacking in perfection in relation to the perfection of Eyn Sof.
We therefore say that afterwards, when the Line came down and made the Residue into Sefirot, it “filled” this “hollow”. This is why the Kabbalistic sages said that the Line “filled” the hollow, and that the entire hollow space was filled with the ten Sefirot of Adam Kadmon (Etz Chayim 1:4).