Problem with imported mesh

Hello I made a 3d stick man model in blender and then i imported into unity but the mesh is transparent, i can see inside it, even with texture. Does anyone know why? And how can i make it look solid?

Thank you.

The most obvious reason is that you have normals pointing the wrong way. See here for a quick tutorial how to flip normals.

Another possibility is that the import is scaled very small so you had to move the camera very close to see it, passing the near clipping plane. If you zoom out, does the problem seem to disappear? You can just change the scale of the GameObject, or select the model in assets viewer, change Scale Factor and Apply.

Last thing I can think of is the shader used is wrong, look at materials, and make sure it's something like the Diffuse shader.