I know this bothers a lot of people, even atheists, but the BigBang can be BOTH the cause and the effect. This isn't a case of Special Pleading, it's a case of Special Circumstances.
Imagine the universe now, and rewind that to the BB. Everything is coming together. All the stars, galaxies, gas clouds, dark matter, energy.... EVERYTHING. Even (and this is where people go cross-eyed) even the SPACE between galaxies. Space itself (what appears to be empty) was pulled into the "singularity" at the BB.
Now, Einstein showed, and it has since been EMPIRICALLY proven (Lorenz contraction, Time dilation, Frame dragging, etc), that space and time are part of the same fabric. You CANNOT have one without the other.
So TIME was pulled into the "singularity" also. OK?
Now... at that exact instant of the BB, at T=0, Space is NOT 3 dimensional.....and time... IS NOT LINEAR.
Spacetime. is. a. POINT. A zero-dimensional point. So anything that "happens" right then, happens EVERYWHERE, INSTANTLY. At that instant, not only is there no Past, there is no Future either.
Now, how we normally distinguish "cause" from "effect" is by checking which happened first. The cause always precedes the effect. Right? But at T=0, EVERYTHING is BOTH a cause, and an effect because there is ONLY ONE point of time. You can equally think of the universe, as "self-caused" or "uncaused".
This ties in with a hypothesis that the entire universe has ZERO energy. The positive energy (which includes matter) is matched by the negative energy of gravitation. Empirical evidence exists for (but does not conclusively confirm) this scenario, the CMB does indicate that the universe is "Flat", meaning gravity and matter are in balance.
So at T=0, positive and negative energy "separated". This does not need an external cause. They caused each other. The manifestation of positive energy "caused" negative energy to manifest. AND VICE-VERSA. And...no "external" energy (or agency) is required.
Furthermore, they MUST occur EXACTLY simultaneously or else there is a violation of Conservation of Energy, and the ONLY place/time that they can "separate" truly simultaneously is at T=0.
This goofy case of causality can ONLY occur at T=0, because there is only one point of space and time. This is also why the Kalam and Cosmologocal arguments fail, because they ASSUME time works like it does now, even at T=0, but we are pretty sure it doesn't with only one spacetime point.
Admittedly this is a hypothesis. I do not assert that this is what happened. Our current THEORIES of the BB break down at 10-43 seconds after the BB. (EDIT: That is to say our theories BEGIN at 10-43 and hold true afterward.) We currently cannot say with certainty what happened before that, but this hypothesis makes a lot of sense to the physicists.