We don't know why spacetime expands. But observational evidence supports that model. There are lots of things we don't know. We don't know why gravity behaves the way it does, or why it exists, or how it exists, but we do know it exists, don't we? We have measured the curvature of spacetime caused by the Earth, and it aligns exactly with what Einstein's GR tells us. We have measured the "Lense Thirring"effect caused by the Earth spinning, and guess what? It also aligns with what Einstein's GR tells us. Einstein and GR, or the BB tell us nothing about any mythical God or spaghetti monster. The standard cosmological model, including the BB is well evidenced and supported. And it will remain that way until someone [perhaps you] can show some evidence to support this God, or even your spaghetti monster. Hope you find him.