The universe, as far as we know. Even if everything in the universe has a pre-existent cause (and that appears not to be the case at the quantum level), it may not be true of the universe itself. "The universe just is, and that's all", as Bertie Russell put it.
But if even proposing that the universe has a reason for its existence is unjustified.A universe which begins for no reason is unjustified and I .aintain it either has a reason or it doesnt and if one wants to remain non committal that state is not expressed in the screaming antitheism of this forum.
If the universe had a beginning and popped out of nothing that equals an argument that the universe is necessary rather than contingent now who would bet on this universe being necessary?
That of course is immediately undone by the question why this universe and not another or even why the u iverse and not a train set.
And yet we find people like JeremyP, Gordon and Bluehillside et cie willing to take a punt on all of this.
If to avoid this you want to appeal to an infinite universe an infinite chain of conti gency doesnt produce a universe.
If I asked to borrow a fiver from you and you had to borrow a fiver from a friend who had to borrow it ad i fi itum you wouldnt get your fiver. If you get the goods its because someone actually had produced one