The Cybertronians were a result of Decepticon rediscovery of Bio-Morphic Reproduction ("budding") ~4 million years ago on Cybertron. The Decepticons were able to rapidly expand their population using this method.
These new Decepticons left Cybertron in the hands of petty warlords unfit for the future empire and took to the stars to conquer other worlds.
In the modern era, the Cybertronian Empire has adopted an aggressively expansionist stance. They claim to have advanced their society beyond the need for war, but hold little regard for non-Cybertronian lifeforms. Under the cause of improving the quality of life for their citizens, they have conquered many worlds, enslaving or exterminating the native population and cyberforming the planets to create "little Cybertrons."
This was apparently under the direction of the Decepticon progenitor known as the Liege Maximo, who rules them from the planet-spanning structure known as the Hub, which harnesses the networked power of the cyberformed planets. The construction of this network was later revealed to be part of a gambit by the Liege Maximo to ascend to the plane of the Dark Gods.
The new Transformers created by budding felt little or no emotional attachment to Cybertron. Primus' plan had shut down reproduction via budding at the intended point and the more these new Decepticons reproduced after that the more thinly his life force was stretched between them, the more they "watered down the line" to use Megatron's phrasing, leading them to be "born" less and less emotional and more and more robotic in nature.
They have a tendency towards a uniform color scheme of light green and silver. Many of the smaller troops must combine in pairs to form one alternate mode (this may possibly be a symptom of their degenerating "genetic" line).
Many citizens of the Cybertronian Empire have long since forgotten the terms "Autobot" and "Decepticon" in the millennia since leaving their birthworld. However their forebears are remembered bitterly by individuals such as Liege Centuro Jhiaxus. By his reckoning the Empire are the "true" Decepticons, having achieved the inborn Decepticon imperative of galactic conquest. He judges the Decepticons and Autobots they left behind as "evolutionary throwbacks." However, given Jhiaxus' former barbarism, he might be projecting his own issues onto his forebears.
A parallel to the birth of the Cybertronian Empire is present within the IDW comics. In this continuity, Nova Prime convinced the populace to charter the Ark spacecraft under a mission of exploration. However, Prime was secretly a Cybertronian supremacist, and with his chief strategist Jhiaxus, his intention was to make the mission one of conquest and colonization much like the Generation 2 Cybertronians. However, this glorious manifest destiny was cut short after the Ark disappeared into the Benzuli Expanse and emerged in the Dead Universe.