Think about PacMan, or the old Asteroids game, where going off one end of the screen would put you on the other side. That's a donut. (The 4 corners of the screen make up the single hole*.) Which "side" is the inside? The question doesn't make sense.
*Edit: as rightly pointed out below, the location of the hole is an arbitrary choice that comes from trying to map the space to a sphere, and does not actually exist anywhere in the space itself.
An interesting experiment is this: imagine yourself existing in the space, which is otherwise empty. PacMan alone in the middle of the screen. Throw a stretchy rope to yourself, horizontally or vertically, catch the other end, and tie it together. Then walk around the space without turning the rope at all. Notice that no matter how you walk around, the rope will always be the same length. Now imagine the same thing on the surface of a sphere. Walking around makes the rope larger or smaller, and there's always a point you can walk to where the rope will completely collapse to a single point.