The United Nations is a government formed by many different governments from Old Earth. It is the sole, historic, uniting factor that many contemporary governments have, and considering the importance of Luna and Mars, many corporations as well.
The United Nations has become its own "nation" since the relocation and the administration of Luna and Mars being undertaken by the organization. The United Nations accepts new applications to be a part of the membership of the United Nations, but only the most influential governments or corporations have a seat as a voting member.
The UN seeks to only administer the Solace System, stating that the home of humanity should be free for all to visit, no matter their choice in new home. The UN views itself as the most secular, responsible, and sensible form of government that the Solace System could be deemed to have, and will not allow any other government take its foothold within the system. The UN allows delegates and citizens from other nations in freely as well as corporations to operate within the jurisdiction of the United Nations as long as they adhere to their policies and are a registered corporation or government. The registration process is very easy and very little is needed to gain approval.
The UN's biggest benefactor is the United States, which has taken a large foothold in the galaxy at large. Some theorize that the United Nations is nothing more than a puppet state of the United States, although the UN would have you think otherwise.