How something can come from nothing.

[Anomalocaris]

Umm, interesting. Well I don't think there was a big bang at all. Why did there have to be a big bang anyway?

Are you questioning whether the universe as we know it had a definite origin at some point in time, or whether all currently known spatial dimensions were at one time tighter then it is now by an enormous factor, and have loosened continuously ever since?

Large scale, systematic relationship between distance and redshift demonstrates convincingly that known spatial dimensions have been loosening for up to 11-12 billion years. Orogen's cosmic background radiation demonstrates those dimensions were much tighter still a couple of billion years before that. This loosening of the spatial dimension of the universe from a very tight state in the past is termed big bang. It is well demonstrated.

If you are questioning whether the initial point of big bang represents the beginning of such universe as we are able to measure and quantify, that is debatable. Some version of theory argues what preceded the big bang is in fact measurable, quantifiable, and accessible. If that is the case, then big bang is not in a meaningful way the beginning of the universe.