According to the big bang theory which is based on general relativity you can wind the Universe backwards making it smaller and smaller until it is a near point. At the beginning of time, everything in time and space was contained in a near point. Since general relativity assigns everything to space-time, everything in that near point is the universe.
It is true that some quantum physicists don't agree with the near point containing everything. They believe that the near point arose out of quantum foam and this quantum foam existed before the universe. However, this quantum foam and it's extreme probabilies and exotic virtual particules is barely understood. Only the weirdest speculative physics apply here.
Outside of space-time, outside of that near point, the rules of math, even Cantors infinities may not exist. Speculative multi-universes including anything outside of space-time remains unproven or non-existent. Unlike physics, philosophy includes the concept of transcendence. Transcendence is the "conceptual" escape from the sensible, from our physical Universe. But beware, the rules of transcendence can never be proved. Outside of space-time everything or anything can be true. Outside of spacetime there is no map and no proofs.
Metaphysics is based on that which is beyound reality. Metaphysics is that book from Aristotle that comes after physics. Within the last 100 years the pursuite of serious metaphysics has disapeared. The last metaphysician from the long list of metaphysicians that I can think of is Whitehead who lived at the beginning of the 20th century. I must admit that I write "about" metaphysics and not directly from the sources. To read metaphysics you have to understand the language or jargon of that particular metaphysician. Metaphysics can be quite technical and difficult. Modern science has taken over the role of being technical and difficult and modern science and the empiricists have won out over metaphysics. Functionally, metaphysics is extinct because it is impossible to prove. Empiricists believe that there are no solid definitions or language that can be used to define metaphysics because it is outside the sensible.
Yet, escape from the universe is not impossible if we make a big leap that we can transcend the physics of the physical/procedural Universe and escape into the metaphysics of substance greater than the Universe. Here there is no solid ground or language or proofs to work with. There are no solid pathways which work in this arena.
Here we are in a totally "psuedo theorectical" zone. This type of theory is separate from proof and experimentation and verification. Real theory exists because it can be validated by observation, experimentation and sensible proof. Yet our psuedo theory by itself can be done totally in the mind or with pencil and paper but is not verifiable per se.
But this does not mean that this transcendent is wrong or a metaphysics is wrong. It means that it is beyond all senses and definition. It is not defineable utilizing the material world or even the proceedural world. It is beyond understanding and there is no sense proof for the transcendent.
However, although it can not be explained empirically, some Rational Idealists think that universals we abstract around us like PI or Space prove there is a world beyond where abtractions exist. When we look at the mirror we see a virtual image which doesn't exist until the lenses of our eyes changes it to a real image. Abstract imaginary numbers placed in formulas bring out solutions which are real. Maybe universals in a four dimensional system come from another dimension.
Maybe universal abtraction in our universe comes from outside the universe and that is what provides a door to the escape from the universe.