Prior to Creation, there was only the infinite Or Ein Sof filling all existence. When it arose in G-d's Will to create worlds and emanate the emanated ... He contracted (in Hebrew "tzimtzum") Himself in the point at the center, in the very center of His light. He restricted that light, distancing it to the sides surrounding the central point, so that there remained a void, a hollow empty space, away from the central point ... After this tzimtzum ... He drew down from the Or Ein Sof a single straight line [of light] from His light surrounding [the void] from above to below [into the void], and it chained down descending into that void. ... In the space of that void He emanated, created, formed and made all the worlds.
-Etz Chaim, Arizal, Heichal A"K, anaf 2