Just because a thing is literally unthinkable by us humans does not mean that it is impossible!
If anything is thinkable, then it is thinkable that things do not need to come from somewhere. You can't have it both ways - either you can use logical inference or you can't. If you aren't using logical inference then anything is thinkable.
There are only two points that we know about this:
1. There are some things that we haven't determined if they have come form somewhere or not;
2. There are some things that we know have come from somewhere.
We haven't found anything that we know just existed, all by itself, without having been caused by something else. In fact, there are so many thing that we DO know have been caused by something else, that it is unlikely that the things we don't know about, simply, spontaneously existed.
The idea of something being outside of the universe is something we cannot comprehend. Everything is part of the universe or else it does not exist... at least that is what our understanding is. Why is our understanding like this? Because we are completely part of our universe.
However, because cause and effect, complex universe, and universal entropy show that the universe had a beginning, there must have been something outside the universe, and before it, to cause the beginning.
Whatever it was that caused the universe, is something that would have to be outside of universal laws. If it weren't, it would simply have been part of the universe. The only way for the universe to have caused itself would be through some form of spontaneous, uncaused action. We have no evidence of spontaneity happening anywhere. There should be at least some dregs of spontaneity around if there ever was such a thing. We haven't found any.
The whole thing boils down, again, to two points. There are tons and loads of cause and effect actions that have taken and are taking place, that we know about. There are some things that we don't know about. A third thing is that, although we can imagine and guess about spontaneous activity, we haven't been able to find any for sure.