My first reaction would be to play some basketball.
After that, though, I would definitely be confused regarding where it came from. But the reason I’d be confused, is because I know some things about basketballs. Most importantly (for our purposes) is that all known basketballs are
made by humans. Materializing out of thin air is very unusual for a basketball. If no further observations could be made, the most I could say about how this unusual phenomenon occurred is, “I don’t know”.
The universes, on the other hand, is something different. The universe is the totality of all we can observe. A basketball is only a part of the universe. From observation we think that the universe is expanding in all directions. With this empirical knowledge, we can deduce that the universe was once much denser. From our observation of the current behavior of matter, energy, and space, we have derived models for the behavior of a very dense universe. I’m not a cosmologist by any stretch, but my understanding is that we can only run the clock back so far before we reach the limits of our understanding. Beyond that we have no concrete understanding.
Nothing we know of today mimics the creation of the universe. We are unable to make any observations aside from computer simulations. So what can we say about it? The most I can say is that I don’t know. I would like to find out, but until we learn more I cannot know.
I noticed you didn’t answer the basketball question. So I’ll put it to you. How would you explain a basketball appearing (seemingly) spontaneously on your table?