Question: What is the universe made of?

Baptiste Ravina answered on 17 Mar 2020:

Here’s a very soothing little video narrated by one my colleagues, Brian Cox 🙂

Today we know that all the matter we see in the Universe, you, me, the Earth, all the stars, galaxies and black holes, only make up about 5% of the Universe. This is incredible! 95% of it, we have absolutely no experience of. This is one of the things that drove me to physics: you get to learn just how much you don’t know, and then try to figure it out by coming up with theories and test them!

Our current theories tell us that the rest of the Universe is largely something called “Dark Energy” (about 70%); “dark” because we don’t know what it is, and “energy” because it provides a very specific type of energy that makes the Universe expand. The remaining 25% are a different kind of matter, which for the same reason we call “Dark Matter”, and I’ve talked more about it there: /moscoviumm20-zone/question/what-are-the-basics-of-dark-matter/ .

There’s also a bunch of radiation going around, mostly photons (the particles of light) and neutrinos (very small particles that travel almost at the speed of light). They only make up at most 0.01% of the Universe, but they actually “warm” it up a bit, to about 2.7°K (degrees Kelvin) – that’s only 2.7 degrees above absolute zero!

Funnily enough, the particle collider I work on, the Large Hadron Collider (LHC) at CERN, is actually colder than that: https://home.cern/news/press-release/cern/first-time-lhc-reaches-temperatures-colder-outer-space