The inflationary period is when the Universe expanded faster than the speed of light, causing local domains to lose contact w/ each other. We live in one such domain referred to as the observable universe as opposed to the total universe.
Things pop into
andout of existence all the time.
It's called Random Quantum Fluctuation
In quantum physics, a quantum fluctuation (also known as a vacuum state fluctuation or vacuum fluctuation) is the temporary random change in the amount of energy in a point in space, as prescribed by Werner Heisenberg's uncertainty principle. They are minute random fluctuations in the values of the fields which represent elementary particles, such as electric and magnetic fields which represent the electromagnetic force carried by photons, W and Z fields which carry the weak force, and gluon fields which carry the strong force. Vacuum fluctuations appear as virtual particles, which are always created in particle–antiparticle pairs. Since they are created spontaneously without a source of energy, vacuum fluctuations and virtual particles are said to violate the conservation of energy. This is theoretically allowable because the particles annihilate each other within a time limit determined by the uncertainty principle so they are not directly observable.
usually
one place they do become real is at the event horizon of a black hole, where 1/2 of the virtual pair escapes and the other 1/2 is absorbed into the black hole, This observable effect is called Hawking Radiation and eventually leads to the total evaporation of the back hole.
One Theory of the Big Bang is that it is at least similar in nature to such a quantum fluctuation.