There. Now you know.

Okay, okay. I think I got this. Folks noticed that some really big items acted differently than their science math said they would, specifically that galaxies behaved like they were much more massive than they really were, and they theorized that there was some invisible extra stuff that only showed up as mass, which isn’t weight but might as well be. It was like the butcher’s thumb on the scale. They called that undetectable cosmic thumb Dark Matter. Then some other people said, no, it’s just that gravity acts different when things are extra heavy. Or, you know, massive. Whatever.