Final answer:
Hydrogen and helium were the first elements to form after the Big Bang due to the simplicity of their atomic structure and the brief period during which the universe was hot enough for fusion.
Heavier elements were formed much later inside of stars. Cecilia Payne-Gaposchkin's work affirmed that stars are primarily composed of these two elements.
Step-by-step explanation:
Hydrogen and helium were the first elements to show up in the universe because they are the simplest and lightest elements. During the Big Bang, conditions were only right for the formation of the lightest elements.
Atomic nuclei formed in a process known as nucleosynthesis, but the early universe was hot enough for fusion for only a short period, meaning only hydrogen, helium, and a small amount of lithium could form.
We understand this from models predicting the conditions of the early universe and from observed relative abundances of light elements that align with the Big Bang theory. Heavier elements only formed later inside of stars.
The discovery that our Sun and stars consist mostly of hydrogen and helium is attributed to Cecilia Payne-Gaposchkin, who against initial disbelief, showed that these were the most abundant elements in stars.