The default address for the MatchMaker is mm.unet.unity3d.com
That will connect a client to the nearest datacenter geographically. However because data centers are siloed from each other, players will only see matches occurring inside the data center they are currently connected to. If a player of your game is traveling to another part of the world, for instance, they may interact with a different set of players that are in that data center.
You can override this behavior by specifying a particular data center. Keep in mind generally as distance grows so does latency, which is why we run data centers spread out over the world.
United States: us1-mm.unet.unity3d.com Europe: eu1-mm.unet.unity3d.com Singapore: ap1-mm.unet.unity3d.com.