If you 'reverse' time and approach t=0, at the Planck era t = 10^-43 sec, Some of our physical laws, such as GR, cease making meaningful predictions; Some others don't.
Quantum Mechanics, and all its effects, still work to some degree.
Even the isotropic, homogenous, hot, dense state, that was present when the Big Bang began, was subject to the Heisenberg Uncertainty Principle. And anything subject to the HUP, by necessity, experiences quantum fluctuations.
These quantum fluctuations would have experienced exponential inflation ( possibly ), and expansion, since that era.
And they would have 'seeded' the large scale structures of the present day universe.
( which is still isotropic and homogenous on large enough scales )