Coat the walls of the room a dark color -- as dark as you can possibly handle -- to create a weightless feeling as would be felt in outer space. Use a flat paint, rather than a glossy one, to add depth. Glossy paint can cause unwanted reflections on the walls. Allow the room to completely dry.
Paint a lunar horizon mural on the wall that reaches a height of a few feet off the floor. Use a shade of gray paint that closely matches the images available of the moon. Extend the mural around the entire room to give the effect of actually being on the moon.
Place a small image of the Earth on the wall a good distance above the horizon to make it appear far away.
Add random stars to the wall or use a star chart and place the stars in their astronomically correct positions. Use stencils or employ free-hand technique with the paint or wall stencils to produce a realistic look.
Cover the ceiling with tiny dots about the size of a pin head by painting them with glow-in-the-dark paint. Sprinkle in some medium-to-larger size dots to give the appearance of stars occupying positions both closer and farther away. Mix some of the glow paint in a spray bottle with water and spray the Milky Way across the ceiling. When the lights go out in the room, the universe will come to life.