Molecules are collections of more fundamental objects, atoms. For example a water molecule, H2
O, consists of two atoms of Hydrogen and one atom of Oxygen. You can create new water molecules by burning hydrogen in an oxygen atmosphere. Likewise you can rip apart water molecules into their constituent parts by passing an electric current through a slightly acidic water mixture.
Now atoms, themselves, are composed of more fundamental objects: protons, neutrons and electrons. Even atoms can be created and blown apart by nuclear type reactions.
What is constant, however, is the total number of protons and neutrons. That sum doesnít change. We know from experiment that as far as we can tell, protons are forever...
As far as expansion of the universe goes, itís an observational fact. Galaxies are moving apart at ever increasing speeds the farther apart they are; this is called the Hubble expansion.
Hope this clarifies things a bit,
(published on 02/08/2008)