I had this theory once that there were huge clouds of hydrogen which slowly congregated together due to their gravitational attraction to one another. Eventually (over billions of years) the size/weight of these accumulations of hydrogen became big enough that massive quantities of heat was being generated. In fact, eventually the heat generated by the gravitational attraction was so much that two hydrogen atoms fused to create a helium atom. This caused a major increase in free energy in the system which then resulted in a chain-reaction of hydrogen fusion....