Electron shells are pathways described by electrons that are orbiting the atom's nucleus (or core). Or in other words, the electrons of an atom move around the nucleus. The movements of multiple electrons can be summarized by an electron shell. For example, the first shell can hold up to 2 electrons and is called the "K" shell. The second shell is called the "L" shell and can hold up to 6 electrons.

The electrons have a negative charge while the nucleus is positively charged. This results in an attractive interaction between the electrons and the nucleus and keeps the electrons on their pathway. The result of the movement of the electrons and this attractive interaction are the electron shells.

The first creation of matter started with the Big Bang. The original universe was very hot and dense and started cooling down. This process led to the exact right conditions to form the building blocks of matter - quarks and electrons. The quarks formed neutrons and protons which then combined into nuclei. Eventually, the electrons were trapped into shells around the nuclei. This process is the formation of the first atoms that then formed everything that we know in our universe.