For many centuries, there were three major empires that ruled the most parts of the known universe: the Kree, the Skrulls and the Shi'ar. Each of these empires had their own galaxies that they commanded and Earth was at the crossroads of the three expanding territories.
In time, the Skrulls and the Kree, who were almost constantly at war with each other, were both greatly weakened or destroyed. The Skrull homeworld was consumed by Galactus, leaving the remaining worlds with no official rulers. Quickly, high-ranking officers claimed certain planets for themselves, each convinced to be the rightful monarch of all Skrulls. Knowing that they were no longer a match for the Kree, some Skrulls manipulated the Kree into war with the Shi'ar. A Nega-bomb explosion killed 90% of the Kree population and the remains of the empire were absorbed into the Shi'ar Empire. Eventually, the Kree broke free of their Shi’ar rulers.
Later, in a reversal of fate, the Shi’ar Empire lost a second war with the Kree and were absorbed into the Kree Empire. However, much like the fate of the Kree, the Shi’ar later freed themselves from Kree control, though the details of how this was achieved have not been revealed.
Ever since the earliest known records of the Shi'ar Empire, they have forced other alien races in space into joining them, whether they wanted to or not. The Shi’ar believe this process to be justified by their religion and, as theirs has grown into the biggest empire in the known universe, they see their belief reinforced by fate. This process of expanding has meant that the Shi'ar Galaxy has also increased over time. The Shi'ar Empire now encompasses easily over a million worlds, though only a select few have appeared within the pages of any Marvel comic.