Every existing object has a cause. If we try to unwind this sequence of causes, we'll get to the beginning of the Universe. But what was the cause of that beginning?
If we accept the Big Bang theory, that means we agree that space-time, nonexistent previously, started to exist. The problem here, as you can see, we can't tell neither where that happened (there wasn't space before), nor when it happened (there wasn't time either).
So we've got an interesting problem here, existence have to have a cause (because existence exists), but the cause can't be found within existence itself (because as we demonstrated above, the cause of existence is out of space-time). Hmm...
What could be the source of existence? As you can see, that was non-existence. Because, when there is existence, it means there is no non-existence. And vice versa. I can't tell these two are reasons of each other, but rather one leads to another. Both make the sequence, which embraces everything that can and can't be thought of, but what about thinker? Is he within the sequence, or out of it?