What does it mean when they say the universe is expanding?

Readers reply: the universe is expanding – but what is it expanding into?, Astronomy

What shape is the universe?

Is space infinite? We asked 5 experts

Expansion of the universe - Wikipedia

The Missing Piece in the Puzzle of Space, by Rushil, TechTalkers

Is the Universe Expanding?

Black holes may be quietly generating the force that is tearing the universe apart, experts say

What does it mean when we say space is expanding? - Quora

New supernova data could help reveal how fast the universe is expanding