There's a great deal of confusion and misunderstanding over 'the Big Bang theory' (BBT) with respect to origins.
Before trying to clear some of that up - beyond what's already been said in this thread - let me point out that there is not one BBT! Rather, there are a great many cosmological models built using General Relativity (GR) and the Standard Model of particle physics (SM), plus various additional elements (maybe). "Big Bang" was used by Hoyle as a term of derision, and, unfortunately, it has stuck.
The class of cosmological models that are usually thought of when BBT is mentioned are based on GR; the "Big Bang" comes from the fact that the only two stable solutions to GR-based models of universes that contain mass-energy are expanding and contracting ones. Hence if you extrapolate back from 'today' the universe becomes smaller and smaller (and hotter and hotter, and denser and denser). The end point of any extrapolation would be a single point ... but that would be an extrapolation too far, in science. Why? Because the two very best bits of physics we have today - GR and the quantum theory which the SM is based on - are mutually incompatible when conditions are sufficiently hot and dense - the Planck regime.
That mutual incompatibility happens well before you extrapolate back to a single point; in fact, you could say infinitely far before.
So, as science, modern cosmological models, being science, cannot extrapolate back 'before' this mutual incompatibility ... unless they use a scientific theory (or theories) which resolve that incompatibility, using a quantum theory of gravity for example.
Actually, cosmological models peter out well before the Planck regime, because the SM is known to be incomplete, and cannot reliably address physical conditions much hotter than ~10 TeV.
However, BBTs do address many origins: of planets, stars, galaxies, all elements other than H, He, and Li, ... and in that sense 'the BBT' does explain the origin of what you 'see' when you 'look at' the Lagoon Nebula!
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