I know that the Hubble constant proves this and scientist have done plenty of research on this topic. Today in my philosophy class we came about the topic of the universe expanding and the philosophy teacher and I were going back and forth on the subject of "How do you really know the universe is expanding." I gave her a pretty good lesson on the scientific facts behind it, but she kept pushing me saying how do you know, how do you know the science experiments are right. My question is, is it 100% fact that the universe is expanding, is it possible that there is a 0.0000001 percent chance that it could not be expanding. Or is it a FACT?