This question seems simple, but its answer quickly gets into some of the most fundamental concepts in cosmology. Let’s start with the Hubble expansion. It is not, as it is sometimes pictured, an explosion in which matter moves through space. Instead, space itself expands, with matter (atoms, particles, and people) carried along like a leaf in a stream.
If the universe is expanding, are atoms, particles, and people also expanding?
Alberto Nyssen, Los Angeles