The largest part of the grid is in fact a gigantic void, known as Void Ocean or Infinite Ocean. Even if it is a gigantic empty place, it is the largest 'thing' on the map.
The void ocean can be defined as the intersim space, where no simulator operates and where no real region (sim) exists. No avatar can exist there and it is impossible for any resident to enter the void ocean.
Still, sometimes objects can be found inside it. They are usually called sim surrounding environment and encircle a sim. These objects really are located outside of the hosting sim. This creates a unique landscape for an isolated sim. Also, the void ocean is covered with water and has a sky. Water is seen with the same color and at the same elevation as in the sim where the avatar is. The same rule applies for sky color and texture.
Void ocean can be divided into 3 main categories: intersim void, void sea and main void ocean. A void sim can be defined as a hypotethical sim, square-sized, full with void, that, at some point, can become a real sim.
Intersim Void is the space that separates sims inside a Sim Cluster or surrounds a Continent. It is large from a single sim to 30 sims. Usually, all void sims are connected one to the other, at least on diagonals. There are a few places where small groups of void sims are isolated from the void ocean. Examples are the gap in North Sansara near the color sims or the two gaps inside Seductive Continent.
Void Sea can be considered a place larger then 30 void sims in a straight line, but with sims at higher distances, both in East and West (or in North and South). Such seas are found in Eastern Ocean or in Southern Ocean.
Main Void Ocean is the place behind every sim. It covers a huge surface, but it is nothing but a large and probably endless void. On world map, there is no limitation to move in any direction. It was suggested that this ocean has a limit. According to Gridsurvey, all sims have Coordinates between 0/0 and 2000/2000, with the grid centered on Da Boom sim (coordinates 1000/1000). So, it is expected that our world, the grid, is in fact a huge square limited to these coordinates. This means that the void ocean covers more then half of the entire grid. See more on Grid Map And Dimensions.
Comparation to real world
There is nothing in the real world with such a sharp change between matter and void. Even the intergalactic void is not creally empty, still hosts a few atoms. The only comparation is done with subatomic world. Electrons, for example, can exist only on certain orbits. They cannot exist between orbits and so they have to jump (teleport?) from one orbit to another. The same can be said also about avatars, that need to jump from one sim to another.
Use of void ocean
The void ocean is very used as a dividing space bebetween sims. This way, residents get more privacy (mainly for residential sims). Even if sims are in contact on their corners, it is impossible from someone to travel from a sim to the other. From ground level, ground and objects of the other sim can be hardly seen only from high Altitude. Sim surrounding environment can sometimes be seen and that is why in many sim clusters there is some distance between each two sims.
Some residents suggested that the intersim void should be used for sailing, but currently it is impossible for technical reasons. A second reason is that many land corporations and their residents will not agree with this. For sailing, see Sailing in Second Life and Water.