Page "Big Bang" Paragraph 17

The Big Bang is not an explosion of matter moving outward to fill an empty universe.

Instead, space itself expands with time everywhere and increases the physical distance between two comoving points.

Because the FLRW metric assumes a uniform distribution of mass and energy, it applies to our Universe only on large scales — local concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion of space.