I can only see the inside of my model.

Your normals are facing the wrong way. You need to fix this in your modeling program, or you can modify a shader to see both sides of an object.

Sounds like inverted normals (which are sort of like arrows hinting that certain sides of your model should face certain directions).

Some modeling programs will accidentally flip one or two normals, while you're working.

If you're seeing all of them flipped, I would guess that something went wrong with whatever export/import process you're using. Which begs the question: what program are you using to make your models, and how are you getting them into Unity?

FBX export plugins are a popular choice. You might check if there are any options having to do with "reset normals" or "reset xforms" in your plugin settings, if that's what you're doing.

I'm using maya. I created my model and then extracted it. But when you place it in unity its reversed.