God. (cheating) This explanation is what everyone eventually resorts to when the other answers fail.
Now, before I go any further, I should make clear that I'm not talking about any of the gods defined in various religions. Throughout all of human history, we've been trying to explain existence.
But nobody has ever succeeded. And it's likely that nobody ever will.
So we create the concept of God, whose only necessary property is being "that which explains existence."
This doesn't tell us much about god, though. It doesn't mean that any particular religion is correct. It merely defines god as something which answers The Question, because we have not been able to.