What is a substance that cannot be separated into simpler substances by physical or chemical means?

Absolutely nothing can not be separated into a simpler substance. Just like tasking someone to reach the number one by counting to one using infinitely smaller components of that number. One can always add a decimal place which gets closer but never achieves it. Science today has identified quarks in matter, although not directly visible at the time of publishing this; however, as technology advances, these too will be broken down into miniscule components.