THE OTHER SIDE: There's more than one of everything. The alternate universe was introduced to us during the season one finale. We learned that while this universe is similar to ours, it has many differences as well. Some people are alive in that universe that aren't alive in ours, or the other way around - people are dead there that are alive here. They are more technologically advanced than we are. Walter took Peter from the other side when his Peter here died, causing a motion that had effects for a long time.