Started about 2 weeks ago with 3D modeling and just finished creating this spartan styled helmet for my characters in Unity. When imported and looking at the face of the helmet, you can see the sky from the backside of the helmet, making the helmet look like it has a hole on the backside. I took pictures of both to show there is no hole on the backside and to show the sky coming through the mask. If any additional information is needed to help answer the question, let me know.
Game engines are real-time engines. And culling the backside of a polygon helps the engine to render things faster. There are no settings in Blender you can turn on or off to control how it is displayed in a game engine. You have to either "solidify" your model in Blender, or find out how to show models double-sided in those game engines.