Half the universe was missing... until now

this episode was sponsored by kiwiko more about them at the end of the show until recently half the universe was missing or hidden or just undetected and no i'm not talking about dark matter or dark energy which make up 27 and 68 of our universe respectively no i'm talking about normal ordinary matter which makes up you and me and the planets and stars and nebulae and basically everything you can see and since most of this stuff is made of protons and neutrons which are forms of baryons this has been known as the missing baryon problem we expect the universe to be made up of five percent baryonic matter but when we go looking we only find two and a half percent now the first question you're probably asking is why should we expect the universe to be five percent ordinary baryonic matter in the first place the answer is because with that density we can explain the relative abundances of different elements that we observe in the universe specifically the ratio of deuterium to hydrogen to helium in the beginning like right after the big bang there were all of these neutrons and protons whizzing around it was incredibly hot and there was tons of radiation the universe was radiation dominated but as the universe expanded it cooled to the point where protons and neutrons could start fusing together a particularly stable nucleus to form would be helium-4 made of two neutrons and two protons the problem was to form helium-4 you first have to form deuterium one proton and one neutron and this is a less stable nucleus and as quickly as it formed it would get smashed apart but by about 10 seconds after the big bang the universe had cooled sufficiently that deuterium could form and as soon as it did it would rapidly fuse into helium the rate at which this happened depended on the density of matter in the early universe the higher the density the faster this fusion could occur then by 20 minutes after the big bang the temperature had dropped low enough that fusion could no longer occur so at this point the elemental abundances were locked in like a snapshot of this moment there was 75 hydrogen and 25 helium by mass which is basically still what we observe in the universe today of the hydrogen ...