Everything which exists, or ever will exist, or ever has existed can be said to co-exist and has emerged from a field of potentiality of which these events form a part. As any event emerges from this field, it negates the potential for another dissimilar event from emerging at that same space and time. I'm describing this non-occurring event as 'non-existence'.
Its qualities could be measured, I suppose, by negation. If event x occurs as 78 degrees, the non-occurring event did not take place in a range of temperature with a range that does not include 78 degrees.
I'm sorry if I appear to be presumptuous, not my intention. I enjoy putting ideas out there to better understand where a possible answer may lay. It strikes me that science is obliged to build upon its own foundations, but philosophy may cast aside preconceived body of knowledge to consider an alternate potential.