Physical laws prior the big bang (quantum fluctuations) A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me. 
What I can't grasp yet is how a quantum fluctuation can even start without an existing universe. Aren't all physical laws created with the universe? I understand that there is no notion of "before" with respect to time, however the big bang is theorised to have occurred, but for that to occur there must have first existed something right?
I wonder also, if there was a more nothingness instead of vacuum before the universe existed and how a quantum fluctuation could have started really from ex nihilo instead of a vacuum.
 A: Indeed, there exist some pre-big-bang theories that arose from string theory. A notable name in this area is Gabriele Veneziano. You can find some information here. This article is somehow technical but should convey the right flavor of these ideas. As you can read from this, there is a two-dimensional space where strings live and a target space, the one we live on, that should start, with inflation. Quantum fluctuations that start the big-bang are happening on the two-dimensional space. These theories have some problems, e.g. the graceful exit problem from the inflationary phase, but are an active field of study.
A: It is possible the number of degrees of freedom of the universe goes up as the universe expands. The larger the volume, the more degrees of freedom there are. A distinction most scientists don't make is between determinism and reversibility. Determinism states that if the universe is in state A at time t, in principle it will be in a uniquely determined state at time t' provided that t' comes later than t. Reversibility states that this will also be so if t' comes before t. Our universe is not classical by any means, but the density matrix describing our universe evolves deterministically. If the number of degrees of freedom is going up, this evolution might not be reversible. The Lindblad master equation for density matrices can handle this sort of thing. If the universe started off from a big bang of zero volume and no degrees of freedom evolving under Lindblad dynamics, there is no reversibility. It is entirely possible that there might not actually be any state at time t' which can evolve into state A at a later time t. Hence, one cannot infer the existence of the past of a state even though one can always infer the existence of a future state.
A: 
What I can't grasp yet is how a quantum fluctuation can even start without an existing universe.

Afaik there is not even a single proof that there was no universe and space before the big bang. Based on the observation of our observable space, we can assume the universe as a whole seems or is entirely flat. If space itself would have been created by the big bang this would not be the case, instead we would observe at least some curvature of space.
Consequently, we can expect that our universe is infinite or at least sufficiently gigantic so that it appears to be flat. If it really is flat, then mass can be created from nothing because net energy is zero, which can happen everywhere in the universe with incredibly low probability. In fact so low, that any interaction of two events is unlikely. 
