The protons and neutrons that compose ordinary matter are made of quarks and gluons. But no one has ever seen a quark or a gluon by itself. Quarks are bound together by gluons. Think of it like this:  Quarks are linked by a gluon spring, a spring that grows tauter as it is stretched so that it cannot normally be broken.  Scientists believe that the tremendous heat immediately after the Big Bang caused the gluon springs to melt so that a quark-gluon plasma would form.  At that instant, quarks and gluon were free from each other. We think that era of unbound quarks and gluons ended when the Universe was 10 millionths of a second old.