For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
From my understanding, this is caused by the universe itself expanding between the 2 objects, not that the object itself is moving that speed relative to us. It’s still completely insane to think about, either way.