Fan Theories volume four

Fan theory four:

Science fiction television programmes featuring humanoid aliens are all set in the same universe as the Culture. The difference is that these programmes portray aliens as humans would imagine them to look like. The events portrayed are after the Culture makes contact with Earth but Humans can't take there being other humans "out there" that look just like them so, in their minds collectively, aliens look a lot different.

This theory also allows for the Drakh and Vorlons to be offshoots of the Culture, or that Babylon 5 is actually anti-Culture propaganda. This reflects the policies of the two races and the mobile nature of their civilisations.