This question already has an answer here:
I'd like to know what is thought of when saying the universe is infinite.
I've read about what is meant when astronomers are speaking of a "flat" universe and I have read that its size is most likely infinite. Yet Google couldn't forward me to a site where it's actually explained what that should mean.
These are some sites I looked but which did not explain it:
- Is the universe finite or infinite?, another Phys.SE question
- Skyandtelescope article
- The wikipedia article on the universe
- Is the universe finite or infinite, an interview with Joseph Silk, on esa.int
- How big is the universe, on www.space.com