Vera Rubin didn’t “discover” dark matter, but she put it on the map.
Dark matter is a wild concept. It’s the idea that some mind-boggling percentage of all the matter in the universe may be invisible, and wholly unlike the matter that makes up Earth. Rubin is celebrated because she forced much of the astronomy community to take it seriously.
That reckoning moment came in 1985, when she stood in front of the International Astronomical Union and walked the audience through some of the data she had collected.