Without it there can’t be all those atoms crushed together and radiating electromagnetic radiation and particles and at such high concentrations and magnitudes.
However, what I just mentioned gravity can do, is due to nuclear forces and their resistance to being crushed by gravity too close together. Eventually, gravity’s squeeze is over-the-line and we get the aforementioned electromagnetic radiation and particles.
I think we’d have to say Gravity and Nuclear Forces are main sources..
I’d give you one, but we don’t know the source of these forces. They are “fundamental” to our current knowledge, but we assume there is an underlying, unifying cause.