Existivity is the first currency of the game. It increases linearly. You start with 0 existivity, which increases 0.01 per second initially.
Existability is a measure of a probability. It increases at a logistic rate to existivity, up to 100%.
There is a button for prestiging just below the currency bar. When done, it will use all existivity to find an existance. That process is not automatically successful, however, and the success rate is equal to existability.