First of all, keep in mind Einstein's famous equation, E=mc^2. Energy equals mass times the speed of light squared. What this equation tells us is that mass and energy are essentially different forms of the same thing. Mass can be destroyed to create energy and energy can be condensed to create mass. The third part of the the equation tells us the relationship between the two forms. The speed of light = 3 x 10^8 m/s. Square that and you get 9 x 10^16 m^2/s^2. That is a huge number. So, it takes only a miniscule amount of mass to release a lot of energy. Now, how does this apply to what goes on in stars? If you take 2 protons and 2 neutrons and weigh them separately, you will find that they contain a certain amount of mass. However, if you then take those four particles and combine them to create a helium nucleus, you will find that the helium nucleus contains a tiny bit less mass than the four particles did when they were separated. This miniscule amount of missing mass was converted into energy when the particles were combined. The amount of mass loss as we create heavier and heavier atoms (and therefore the amount of energy released) is by far the greatest for the fusing of hydrogen atoms to created helium. With each new element created, the amount of mass loss gets smaller as the new element gets heavier until we reach iron. Iron is the break-even point. We lose no mass in the creation of iron and, therefore, generate no energy. All through out this process, it is only an ever decreasing amount of the mass that is lost. It's a miniscule amount to begin with and it just gets smaller and smaller from there. So, Stars will never be able to convert their entire mass to energy. So, the universe will never run out of matter because of the activity of stars.