The Amenthes

Also called the Duat, the Amenthes are a vastly technologically superior race to the Raman Empire. They do not recognise the Empire as an independent state, but also have no interest in dealing with (or conflicting with) the Empire. Their ships travel at very high warp through Raman space to destinations far outwith the Empires knowledge.

The Ramans understand that the Amenthes are content to ignore them, and early attempts at stopping the Amenthes ships have proved to the Ramans that doing so is futile.

The Amenthes technology is tens of thousands of years in advance of the Ramans. There has never been any direct communication between the two, so much so that even the name Amenthes is simply what the Ramans have named them - they have no knowledge of the races true name. 

Amethes technology allows for the folding of localised space. i.e. They are often much bigger on the inside. This means that Amenthes ships are usually very small on the outside (a few cubic meters) but huge on the inside. 

In most sci-fi shows all of the races are roughly on the same level technology wise (except with the notable exception of the Vorlons in Babylon 5). This is unrealistic. If civilisations are springing up across the universe then there would be massive differences in the age of the civilisations.

What if the dinosaurs had invented space travel, they would be millions of years ahead of the Federation. 

The Amenthes are so far beyond the Federation and Raman technologically that they see no reason to converse with either. In fact they probably are unaware that the Federation is a separate entity from the Empire.

In reality the entire Empire is contained within what the Amenthes consider their dominion. Think of the Empire as an Amazonian tribe with no contact to the outer world. They believe that they are masters of their surroundings with no idea of the parent state Brazil or it's millions of people.