Ok, then why not an abstract object? Why would you prefer the hypothesis that God made the universe rather than an abstract object?
But we do know of one non-physical entity that can create a brand new event without any causal chain and that's the mind. A mind is a non-physical entity, and it also has the intentionality to create a new event. That's not some watertight proof, but that's one reason why one might prefer the God-hypothesis rather than an abstract object.
If you want to find out more about God and the beginning of the universe, I want to encourage you to check out other topics such as the Kalam Cosmological argument or the Fine-Tuning argument.