system
Given the following post, respond as if you're commenting on Reddit:
human
Why was early universe made up of H and He? Were there any heavier elements?
gpt
A bit of context first: Atomic nuclei join each other fusion, and that takes a lot of energy to occur. Take two or more "small" atomic nuclei (like hydrogen, say), crush them together under extreme temperature and pressure, and they'll transform into a helium nucleus. Those atomic nuclei are made up of smaller bits - protons, neutrons - that are in turn made up of smaller bits - quarks, gluons. 

The Big Bang created a soup (called a quark-gluon plasma) of the parts that make up atoms, not atoms themselves at first. As that soup cooled and expanded over the first few minutes, those parts came together to form the simplest atoms - hydrogen and helium... and a tiny bit of lithium and beryllium (the next heaviest elements) too. 

Over time and as stuff continued to cool, the hydrogen came under the influence of gravity to form local areas of higher density gas, then gravity sucked in more hydrogen. The centre of these clouds became hot and dense and under pressure enough for the hydrogen to start the process of fusion...and stars were born. Hydrogen transformed into helium, then helium fused into lithium, and lithium fused, and so on until the star couldn't do this and remain stable. Some of those stars eventually reached a point where they effectively exploded in novae and supernovae, and this hugely energetic explosion created enough energy to  fuse even higher elements to create uranium and other atomic elements with large numbers of protons.

**TL;DR**: The universe started out as a fog of dissolved subatomic particles. These first came together to form largely one-particle atoms (hydrogen). Then the hydrogen collected into stars, and their fusion results added other heavier elements later.