"Island Universes: Galaxies are the lighthouses that plumb the Universe - constituents of the largest-scale texture we know.
(...)
Our appreciation of the universe beyond the Milky Way is entirely an achievement of the twentieth century. (...) By the 1920s, photography had revealed that there must be tens of thousands of these objects, by then known as white nebulae to distinguish them from the clearly different gaseous nebulae such as the famous Orion Nebula, accessible to the telescopes of the time. They showed a variety of spiral, elongated, or oval forms. The most plausible theories to account for these nebulae made them either nearby objects - perhaps planetary systems in formation - or extremely distant, truly "island universes" of which our Milky Way, hitherto the entire known Universe, would be merely one among myriads."
http://www.astr.ua.edu/goodies/data_resources/galaxies.text
