The 80% number comes from the Dark Energy Survey (https://astronomynow.com/2017/08/05/survey-reveals-most-accurate-measurement-of-dark-matter-structure-in-the-u

The 80% number comes from the Dark Energy Survey (https://astronomynow.com/2017/08/05/survey-reveals-most-accurate-measurement-of-dark-matter-structure-in-the-universe/ https://astronomynow.com/2017/08/05/survey-reveals-most-accu...): DES scientists used two methods to measure dark matter. First, they created maps of galaxy positions as tracers, and second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of dark matter over billions of light-years, using a technique called gravitational lensing. To make these ultraprecise measurements, the DES team developed new ways to detect the tiny lensing distortions of galaxy images, an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals. In the process, they created the largest guide to spotting dark matter in the cosmos ever drawn (see image). The new dark matter map is 10 times the size of the one DES released in 2015 and will eventually be three times larger than it is now. There's more info here: https://news.fnal.gov/2021/05/dark-energy-survey-releases-most-precise-look-at-the-universes-evolution/ https://news.fnal.gov/2021/05/dark-energy-survey-releases-mo... And here: https://www.darkenergysurvey.org/ https://www.darkenergysurvey.org/ Dust accounting for the missing 80% of matter and made out of normal matter would be visible in these surveys. It would block, reflect, or emit light. But it isn't visible despite bending light around it—and that's the dictionary definition of dark matter.