Dust in Galaxies: Clumps Instead of Haze
Light from a distant galaxy reaches us through its own dust. This dust is not a smooth haze but ragged clouds, like wisps of cotton. Old models treated it as a uniform mist, and then the galaxy didn't add up energetically: too much light disappeared in the ultraviolet and too little came back in the infrared.
Astronomers did what a cook does when they dice a potato instead of tossing it in whole.