whether there is more universe past the observable universe or not, if matter is finite then there is a "last" galaxy if you travel in one direction until you reach it. looking back in the direction you just came from you will see the galaxies you passed by. but if you look in the other direction you will see nothing but blackness (empty space).
would going into into this matterless portion of 3D space just go on forever and you will never come across matter again? and light as well once you travel far away enough