Put your hand in an empty bag, and use it to pick something up. The bag now contains something, which was always there, but which is now inside the bag.
Personally, my theory is one where there are a multitude of universes, which sometimes spill over in to each other, either through black holes or big bangs. Oh, and dont forget that there are also likely to be many dimensions that we cannot see nor understand, which could also explain where stuff came from.
Sorry, but more science than philosophy, maybe not phrased in a scientific way.
If you have a bag, it has an inside. If you turn it inside out, it still has an inside, but it is not the same inside as before.
Brane theory is something which indicates that there are more than one universe (multiverses) which exist, and from time to time, bits can spill over from one to another. In the case of the big bang, some schools of thought theorise that matter from one brane spilled out in to ours, in essence, the effect would be the same as a bag being used to grab something, then being turned inside out, so the contents which were grabbed, whilst previoulsy being outside the bag are now inside it. The stuff came from a different brane, or universe, but have not just "appeared"
From nowhere at all, if you follow the quantum reasoning. Tings at sub-microscopic level get far weirder. Particles being in two places at once, popping in and out of existance. And weirdest of all particles changing their behaviour according to whether you observe them directly or not. As though we were all the product of our own imagination. At the same time we're made of particles. It's like, if you add enough zeros up you end up with a number.
OK one more time ... Where did the particles come from. And.... There had to be an original Universe, before the multiverses. What I'm trying to say is that whatever the nature of being at the moment, it had to start somewhere. Why and How is probably beyond human comprehension.
Sorry I'll get my coat.
According to quantum theory particles can appear out of nowhere, but this should be in pairs, e/g/ 1 matter, 1 antimatter and they would normally both disappear at the same time as each other. However, quantum theory can be used to explain anything which it is not possible to explain any other way, e.g. a particle being in two differing places at the same time. I think that strings can do even more clever things, if you ask a physicist to explain he will tell you that it is too complex to do so. In other words, trust me, I'm a nerd.
According to a book I once read, the world is a giant flat disc being carried on the back of four elephants that in turn are standing on the back of a giant turtle that is slowly making its way across space.
It gets more mind bending when you think that on a basic level, everything, everywhere is made of the same stuff. We are all Protons, Electrons and Neutrons. It's just the arrangement that's different. You and your teapot on an atomic level are identical. Mad isn't it?