“No competent thinker [can] maintain any single nebula to be a star system of coordinate rank with the Milky Way.”
–Agnes Mary Clerke, Irish astronomer, founding member of the British Astronomical Association, and honorary member of the Royal Astronomical Society, commenting in 1890 on the likelihood that other galaxies existed beyond the Milky Way. This comment was captured in an excellent article from the July 31, 2021 Science News, “A Century of New Worlds,” which discusses discoveries in astronomy over the last 100 years. The article also notes that it was the work of another female astronomer, Henrietta Leavitt, that helped to eventually prove the existence of other galaxies.
Extra: A crater on the Moon is named after astronomer Agnes Mary Clerke (shown below).