"Nothing" is not something.
However, we do know (from experiments) that "nothing" is unstable and will eventually break down to "something" (and the corresponding "anti-something" thus keeping things balanced)
The Big Bang theory is not about "the start" of the universe. It is, more simply, a theory based on the hypothesized effect of the expansion of space on the energy content of the universe:
Same amount of total energy + more space to spread it out = the energy density goes down with time.
When we use the theory backwards (to try and understand the past), we note that the universe was much denser (and hotter) in the past. When we look very far, we see in the past and, sure enough, the universe was denser back then.
At each critical level of density, things change. The same way that if you take ice and you keep heating it until its temperature goes above 0 C (32 F)... it suddenly becomes liquid (it changes state).
The energy does the same thing. Above a certain temperature, the electrons cannot remain in orbit around protons, and the universe becomes opaque ( = not transparent). Higher still and protons cannot exist (the quarks get knocked around by the energy density), and so on until no matter can exist, none of our common forces can exist and...
there comes a point when we do not understand how gravity and time can work. This density is called the Planck density (from the name of a scientist who calculated such things) and, if you follow the calculations of the Big Bang model, existed around 13.8 billion years ago (13,800 million years).
That moment is called the Planck Time. Because we do not know how time works at that moment, we cannot use the word "before" at that moment. It does not automatically mean time did not exist. For all we know, the universe could have been expanding eternally "before" that moment; after all, the Planck density is NOT infinite (there is no singularity within the domain of the Big Bang model, only "before").
At the Planck Time:
-- space was already expanding,
-- the initial energy already existed,
-- the density was not infinite (it is a number that has been calculated)
-- matter did not yet exist (it was too "hot"); matter comes later.
and this last one is conditional (it includes "if"):
-- IF the universe is presently infinite in size, then it was already infinite in size at the Planck time.
I think that, one day, someone will understand the link between gravity and time (someone will "unify" the two into one "thing" that we do not yet understand) and we will be able to understand further back (until we hit the next wall).
Hawking had imagined "particles of time"; working with spacetime (the 4-dimension space where time is one dimension), he imagined an "instanton", the smallest particle of spacetime. At the exact "time = 0" (just "before" the Planck Time), this instanton was a special particle from which all time directions were "future",
the same way that the North Pole is a special geographical point from which all directions are "south". That is what he meant.
On Earth, you cannot go north of the north pole,
in spacetime, you cannot go before the "before pole" where all directions are "future".