The Principle of Sufficient Reason
Nothing just is. Leibniz's rule: every existing thing owes you a reason it exists. That cracks the cosmos.
Grounded in the research on the principle of sufficient reason
Demand a reason
Start here: refuse to accept any brute fact. Leibniz called it nihil sine ratione, nothing without a reason. The Principle of Sufficient Reason says everything that exists, every fact, every truth, has an explanation for why it's that way and not otherwise. He laid it out in the Monadology, sections 31 to 32, in 1714, naming it one of his two great principles next to non-contradiction. Your move: when something exists, ask why.
Sort necessary from contingent
Split everything into two bins. A necessary thing exists by its own nature; it couldn't fail to exist, so its explanation is internal. A contingent thing could've not existed; it needs an external cause. You, this planet, the moon, every atom, all contingent. Run the test: could it have been absent? If yes, it doesn't explain itself, and the PSR says something else must.