Just in case anyone of you don’t know, or are curious. The term island universe was originally coined by Immanuel Kant before he was a philosopher. He was reading an article from some fundie named Wright. With the ancient Greek arguments “spheres are perfect, so the universe must have such structure” mentality, he thought that the Milky Way was a giant sphere, hollow in the center, but Earth was on the edge, so it looked like Milky Way was edge on. Well, Kant, correctly misinterpreted (luck him 🙂 ) the article to mean that the Solar System was on the edge of a disc which is the Milky Way. Unfortunately, most went with the hypothesis that the spiral nebulas were places in which star systems were being formed.
It wasn’t until Edwin Hubble in 1925 that the spiral nebulas were confirmed to be more island universes other than the Milky Way. He did it by finding Cepheid variable stars. Those stars change brightness as they change size, using them as reference point, one can determine the distance to such place. The way it works is that if you know its absolute magnitude, then you can determine its distance, since the more distant it is, the dimmer it is. Not only is the fact that he found stars in the galaxy counter to the claim that it was a place of star formation, but it was measured to be 1,000,000 light years away! That is so far out that it couldn’t have been in our galaxy. After the way it was measured was calibrated, the actual distance is over 2,000,000 light years away! The universe is much larger than we could have ever imagined.