Atomic nucleus
As the Universe cooled after the big bang  it eventually became possible for particles as we know them to exist.  The most common particles created in the big bang which are still easily observable to us today were protons (hydrogen) and electrons (in equal numbers).  Some heavier elements were created as the protons collided with each other, but most of the heavy elements we see today were created inside of stars during a series of fusion stages, such as the proton-proton chain, the CNO cycle and the triple-alpha process.