You are in a hot, dense place. You are surrounded by nuclear reactions that are rapidly fusing hydrogen into helium. You notice that your surroundings are cooling and rapidly dropping in density. Within about 5 minutes, the fusion reactions stop. Where are you?

You are in the era of atoms in the early universe.

Explanation:

A nuclear reaction occurs when light elements combine into heavier elements (that is known as nucleosynthesis). There are different ways in which that can happen, Proton-Proton reaction, CNO cycle.

3 minutes after the big bang the early universe reaches a temperature at which was possible the fusion of proton and neutron to form atom nuclei.

The cooling and drooping in density mean that the first atoms were able to form since the atom nuclei capture free electrons.

Therefore, you are in the era of atoms in the early universe.