Is is possible to make hollow space?

In Unity, the regular sphere/box/capsule colliders are set to solid insides. But a Mesh Collider can keep things in. In the FBXimport panel, you can check GenerateCollider(?) to get one (or can probably add it later under Components.) Be sure not to check Convex (a speed-up for meshes that are mostly inside-in deformed cubes/spheres.)

If you only want to keep things in (no one will ever jump on it,) Unity keys off the mesh normals to decide in/outside of a MeshCollider, so you'll need to flip the normals in Blender. Now dropped stuff will fall through the top, but stop on the inside bottom. Unity wants to cull (skip drawing) backfaces (which are usually inside the model and never visible anyway.) So, an inside-out mesh will look like it has transparent outer surfaces. Simplest is to modify a shader not to cull backfaces, it you want to see it from in and out.

If you modelled a "double-boiler" sphere (check the inside wall normals point in,) I'm guessing a mesh collider would work for that, too, and it would act like a hollow metal sphere. I'd make two similar sized spheres, delete verts from each, and weld the open edges together, but I'm not a modeller.

I'm not totally sure what you actually mean, this seems to be a modeling question. You would yield better results by posting maybe on blender orientated sites imo.

Anyway i'm guessing you want to make the inside of a sphere visible, if that is the case then you need to flip the 'normals' of the mesh. I have no experience in blender though to explain how to do this unfortunately, however in 3ds max, it is just simply a click of a button.

Good luck!

Ive worked with blender before, and if you flip the normals on a cube for example, then when importing into unity you should be able to go inside that object or just create an object with an "inner" and "outer" layer of mesh. Eiher should work

Also, in unity you can create a large collide and place your objects inside it.