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.
I may be wrong but I thought it was generally believed that the Universe is infinite (or at least that that was the most common belief among those that are qualified in the field)
Edit: I mean infinite in Space, of course it hasn't existed forever
Edit2: I quickly read this article https://www.swinburne.edu.au/news/2021/08/Is-space-infinite-we-asked-5-experts/ where the 3 astronomers answered maybe, yes, and yes. Whereas the two non-astronomers answered no. Since this seems to agree with me I will believe that this is correct and not investigate further