How big is our #universe?
In 1920s astronomers believed that our Milky Way Galaxy was the entire universe.
The answer came with the detected variation of a single spot in the #Andromeda#Nebula, when Edwin Hubble noticed that the spot varied, and on wrote "VAR!" on the plate.
The best explanation, #Hubble knew, was that this spot was the image of a variable #star that was very far away.
So M31 was really a #galaxy possibly similar to our own.