Question: what is the universe made of? @ciaran

Ciarán O'Brien answered on 18 Nov 2014:

Tricky question, because we can’t actually see about half the stuff in the universe. It’s called dark matter and we only know it exists because it affects the stuff we CAN see with its gravity. We’ve no idea at all what it’s made of.

The half we CAN see, it’s mostly hydrogen. It’s the simplest element, so it was the first and fastest to form from the heat and subatomic particle soup of the big bang. Next most common is helium, which was mostly formed by the first stars (They’re bright because they convert hydrogen into helium using nuclear fusion) exploding and spreading it across the universe. There are heavier elements too, in ever-decreasing amounts, for the same reason, but hydrogen and helium make up 99% (maybe more than that, even) of what we’re able to see of the universe. 🙂