Many apologists and creationists have this strange notion that if nobody can present an alternative explanation for the origin of the universe, then their God hypothesis should therefore be accepted by default, until something better is presented. In fact, many apologists actually present this very argument in all seriousness.
It shouldn't be necessary to emphasize the fallacy in this kind of thinking. Even if there were no alternative explanations whatsoever, no alternative hypotheses to rule out, that doesn't make such a made-up explanation as "God did it" any more plausible. Having only one hypothesis for something quite obviously doesn't make it somehow automatically true or even more believable. It's not like it gets an exemption for being the only one.
But if we wanted to present an alternative hypothesis, just as a thought experiment, and just to show that it's not outright impossible to come up with such an alternative, consider this one:
Our universe resides in a kind of "meta-universe" (or "metaverse" for short) whose properties might be completely different from the ones inside this universe of ours, and nigh impossible for us to fully comprehend. (Think of quantum mechanics, up to eleven.) Our universe is a kind of "bubble" inside this metaverse.
Concepts such as space, time and causality might be completely nonsensical in this metaverse. They just don't work in the same way as inside our universe. If you want, you could hypothesize that the metaverse might be "spaceless" and "timeless", and that things like causality are really fuzzy things (a bit like in quantum mechanics, but at an even more incomprehensible level.)
There could be a physical mechanism, like a law of nature, in this metaverse that constantly pops up randomized universes into existence, like bubbles appearing in boiling water. This mechanism is just a simple physical mechanism, just like gravity or entropy in our universe. It doesn't have any kind of mind or goal, it just is. It's simply an intrinsic property of the metaverse, just like eg. gravity is an intrinsic property of our universe. (We could further hypothesize that it always creates universes in pairs that cancel each other, one universe always being the exact negative of the other. Balance seems to be an intrinsic property of everything, so it would make sense that entire universes are also always balanced in this manner.)
These universes might or might not have a limited existence.
Each universe has a random set of energy and physical properties. In the vast majority of these universes the amount of energy and the particular physical properties are such that they never develop any kind of life (if they develop anything at all; they might just remain devoid of any structure for their entire existence.)
In the case of our particular universe, it just happened, by chance, to have the exact amount of energy and physical properties for our kind of life to form. There was no higher purpose or goal for this universe to be like this, it just happened to be like this from among innumerable random universes.
Now, the point of this hypothesis is not for it to be a correct explanation, especially since there's zero evidence for it. Its point is to show that the God hypothesis is most certainly not the only possibility. It's easy to come up with other explanations.
Of course if you present this hypothesis to an apologist, he or she will most probably, and quite ironically, immediately object with the same objections that are presented against God: Where did the metaverse come from? Where did this universe-popping mechanism come from? Why would you believe in this hypothesis given that there's zero evidence of it? And so on.
The irony here is, of course, that apologists want to exempt God from being subjected to all these questions, while not having any problem presenting them about other hypotheses such as this one. Anyway, as said, that's not really the point. The point is that there are other possible hypotheses. The God hypothesis is not the only one and does not have any special status.