We can ask of any particular thing why it exists, but we cannot coherently ask of existence why it exists. Since existence requires no explanation, questions as to when it began, and by whom it was created, don't make sense. Since no explanation of existence is necessary, no questions can arise about it.
Existence is simple, irreducible, and foundational.
It is the bedrock, the utterly basic, that provides the context for all other explanations. It follows from the Objectivist metaphysics that God , as understood by Western monotheistic religions, does not exist. Atheism is the only rationally defensible attitude toward the question of God's existence. Some people think that if there is no God, then life has no meaning or that there is no reason for a person to be moral. Rand dealt crushing blows to both objections.