We have very solid calculations about the density of the Earth, and those are backed by a multitude of observational data. The Earth is almost certainly not more hollow than we think.

As for life existing inside the Earth beyond the Earth's crust. It is not impossible, but it is very unlikely, and there is no evidence to suggest it is true. For example, no unknown organic compounds show up in magma samples that come from deep underground and no cells have ever been discovered.