@BowsacNoodle @wgiwf @SuperSnekFriend @merchantHelios @teknomunk People did that before him I think, Edward Barnard was probably the first to consistently do it in the early 1880s. What he did was use the largest telescope at the time (The Hale telescope) to study something called 'cepheid variable stars'. Keeping it simple, types of stars have a easily predictable brightness, so he could make an estimate of the distance to them. The measurement showed that all of these nebula were significantly further away than pretty much anything else we knew of, suggesting they were outside the Milky Way.

The images below are of Barnard, vs the one Hubble used.