Complete Dr. Aris's dramatic lab journal entry about his recent mishap.

It was not until the purple smoke filled the entire laboratory _____ realized I had mixed up the baking soda and the highly reactive unobtainium.

The correct answer is that I.

In an it-cleft sentence focusing on a time phrase (especially with "not until"), the relative clause must be introduced by that.

While "when" might sound tempting because it refers to time, cleft sentences rely on "that" to connect the focused element to the rest of the clause. "Did I" would only be used if the sentence started with negative inversion (Not until the smoke filled the lab did I realize...).