Comment intergalactic medium (Score 3, Informative) 53
Aside from this direct observation there's another source of evidence for a lot of matter outside of galaxies.
"The analysis revealed a striking picture: roughly 76% of the universe’s ordinary matter is spread thin in the IGM."
https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fwww.sciencenewstoday.o...