As all stars are converting mass to light/photons, and i guess giving them out faster than the universe expands, why is it that the universe is not getting lighter?

Because light can't leave the universe.

Because light can't leave the universe.

More luminous.

They are not being given out faster than the universe expands.

Because the photons "weigh" the same as the mass that was converted into them.

First, Its not exactly that the sun or stars turn matter into energy, it fuses the matter (hydrogen to helium for instance) thus releasing stored potential energy. It's like rolling boulder down a cliff, it can do work, but you dont turn the boulder itself into energy.
Consequently to fission helium into hydrogen, energy is required. This is like water and electrolysis. You combust hydrogen and oxygen, the reaction releases energy, but the hydrogen and oxygen isnt destroyed, it changes form, becoming water. If you pass a current through the water, you can seperate the hydrogen and oxygen, using up electrical energy at the same time.

Now these photons which are created interact with other matter, thus being converted into heat, or initiating a chemical reaction (how plants create sugars) etc..
So basicaly, they are 'used up' as fast as they are created. Same reason when you turn on a light bulb the room doesnt get steadily brighter while its on, the light is absorbed by the objects in the room.

-Andrew