Basically, I'm thinking that the Humans, while not necessarily the most technologically advanced race, have proven to be really good at exploring and adapting, and as such can be found in pretty much every corner of known space. Although one of the relatively newer races on the galactic scene, they've spread at a prodigious rate.
Of course, they kind of had to, ever since they blew up their homeworld. Now there's just a second asteroid belt in the Sol system.
One thing that the alien races quickly noticed about the humans is that they generally just seemed to be quicker to band together and draw swords than some of the other races- let's face it, humanity has a military history that runs VERY deep. Once humanity didn't have a homeworld to call their own anymore, they also didn't have the same territorial borders that they had fought over for thousands of years. As such, as new factions emerged, they were divided by ideas and principles and goals rather than by region, in most cases. This has resulted in everything from 'nations' of artists of various types, to other 'nations' of soldiers, hedonists, white supremacists, pretty much anything. The aliens, seeing this divisive nature (along with the human's readiness to violently defend their ideologies) wound up coming up with their own name for the humans. Though the pronunciation and exact words vary from species to species, translated into english the humans are frequently referred to as "The War Cults", or "Cultists" as an individual. It's become a blanket term to the point where even those humans not affiliated with such a faction are often referred to as Cultists.
Due to the rate at which the humans spread and made names for themselves, there is quite a lot of power in the collective hands of humanity. However, the divisions between factions have only deepened since Earth was destroyed ('Spacefall', as they generally refer to it) and so the odds of them ever banding together are effectively null. That's probably a good thing, since by now humans could be said to have the largest fleet in the galaxy, if not the most technologically advanced- the key phrase being that they COULD be said, because this fleet will never act as a single fleet.