Keith A says:

"There is very little matter between the galaxies.
The galaxies are clumps of matter within and from which stars formed."

Surprisingly, this isn't right. Even if you ignore the putative dark matter, if you look at a cluster of galaxies, there is about 6 times as much mass in hot gas (mainly hydrogen and helium, but also rather surprisingly including heavier elements) between the galaxies as there is in the stars in the galaxies themselves. We know this because the gas is hot and emits X-rays, which can be observed by X-ray satellites. But its only been known since about the 1980s.