Dennett throws down the gloves and pummels Sam Harris right in the gut

[pocaracas]

Um, how about Big Bang cosmology? Black holes?

The universe began to exist therefore something caused it to exist,

Are you sure that's how that sentence goes?
Did it begin to exist?
Or does it exist since the beginning of time? Thus it has always been in existence?

Do note the careful use of temporal words here... without time, there's no causal effect.

and the universe can't be the the cause of itself of course as it would have to exist in order to make itself exist in the first place.

Like I said above, no Universe, means no time and no space. No time means there's no such thing as a causal effect, so no cause for the Universe's existence.
I know it's difficult to think about absence of time, since all our verbs rely on the existence of time, some way or another...

So you will need something eternal and fully self sufficient within itself that would bring space, time and matter into existence.

hmmm... quantum fluctuations in vacuum seem to fit the bill...
Do you think we should be so bold as to assume it to be something more complex than sporadic quantum fluctuations which bring about a few particles, every now and then?