A SID is not a GUID, it is not intended to be huge unique identifier with trillions of unique values, it is an arbitrary length STRING containing various information which includes the domain ID and the relative ID.
For example, S-1-5-7
is the SID for Anonymous users, S-1-5-32-545
is the SID for the local Users group and S-1-5-32-544
is the SID for the local Administrators group. These SIDs are common to all installations and therefore known as well-known SIDs. See: http://support.microsoft.com/kb/243330
As for who assigned your domain ID, it is randomly generated when the domain is created. It is only unique so far as a new domain will not use an ID already in use within the Forest, but there could be hundreds of domains out there with the same ID.