Sorry to give you this type of answer but nothing, the edge of the universe is expanding at the speed of light and since only light can go that speed even light wouldn't be able to catch up and impact the "wall" so you can never see or touch it. There would be nothing to touch. Theoretically there is nothing obstructing your hand from going any further but because it is the edge of the universe there would be no space to put your hand in so you can only wonder.