In 1924 Edwin Hubble (after whom the Hubble Space Telescope was named) proved that there were indeed galaxies outside the Milky Way. The groupings of galaxies in the sky were so obvious that the existence of clusters of galaxies was accepted immediately. The existence of superclusters is less obvious. Finding the distances to galaxies, and their three dimensional distribution in space is difficult. Most astronomers accepted Hubble's word that superclusters did not exist. Clyde Tombaugh, who discovered Pluto, once showed Hubble his first map of a supercluster. Hubble refused to believe it so the idea was ignored.
Gerard deVaucouleurs was not so easily discouraged. In the early 1950s he suggested that our galaxy and its cluster, the Local Group, are at the edge of a much larger group. This larger group is now known as the Local Supercluster. DeVaucouleurs' suggestion was initially "received with resounding silence" (his words) but after 25 years his view became widely accepted.
By the 1970s, improvements in instrumentation allowed astronomers to measure distances to a large number of galaxies and work out the three dimensional structure of the universe. The first definite supercluster was mapped out by Stephen Gregory, Laird Thompson, and William Tifft in the 1970s. They found the Coma Supercluster extending about 100 million light-years. Two clusters, the Coma cluster and A1367 (the 1367th cluster in a catalog by Abell), were bridged together by galaxies and small clusters. The trio also unintentionally found the first void in front of the Coma supercluster. Astronomers quibble about the details of superclusters but most accept the fact that the large scale structure of the universe includes superclusters and relatively empty voids.