Multiple Dimensions are created when any decisions are made in any dimension. This means that billions of different dimensions are being created every millisecond. The Prime Dimension is seen as the main dimensions as it is one of the few that does not end in total destruction of the universe. Portals can be built that allow for inter-Dimensional travel however only if the dimension on the other end has created the same portal during their timeline. In simplified terms dimensions are seen as 'what ifs?' where you can see the outcome if a something different had happened in time, for example what if GreenFury used his powers for evil instead of justice? There are many dimensions where this has happened and they have ended in many different ways. Sometimes dimensions can be damned which means that they only last up to a few seconds before collapsing in on themselves, this can happen for a multitude of reasons.