" It is an intrinsic expansion, so it does not mean that the universe expands "into" anything or that space exists "outside" it."
To my understanding conceptually is the grid, a structure that in a two-dimensional space is made of intersecting lines used to organize and divide space, that is expanding and as it expands so does the distance between any two points in space. The distance between its infinite points that once was zero turned up to be increasing positive. No points from outside are moving in.
