What forced opposes the explosions in the sun during the fusion to keep the sun stable and balanced?

It's all about gravity. Every atom exerts a gravitational pull on every other atom. With enough atoms pulling on each other, the matter is compressed into a very small space. This generates a lot of heat, causing individual atoms to fuse together and release energy in a manner similar to an explosion. This energy release creates an outward pressure that balances out the gravitational pull. The star remains stable for as long as it can keep this balance.