They have always been there.

Literally. Ever since the beginning of this particular universe. I could tell you what they called themselves before their ascension, the name of their planet and of their star, but it would be useless. These don't exist anymore. 

The race the became the Shadows arose when their universe was dying. The few stars that remained had long since grown cold and dim - they held no promises, no hope of a sprawling empire. Only a slow death awaited. The Shadows didn't accept that. They felt cheated, robbed of their birthright. And in a moment of brilliance and madness, their scientists found the answer - if this universe was gone, they would have to plunder the next one. All of their remaining resources were thrown into this sole objective, and when at least the last star went out with a wimper, it paid out. The Forever Shadows rode the Big Bang that somehow followed and watched as galaxies were born in front of them. They could finally build their empire.

Except... Their forms were now immaterial. They couldn't manipulate tools and weapons as they could before. And on top of that, the laws of physics had gone slightly askew from what they had known - their science and technologies weren't of much use anymore. They grew enraged and bitter at that development, surely, but also doubled down on their ambition. The Forever Shadows were persistent more than anything - nothing lesser could be expected from the race that survived the end of their own universe. So, they stepped away from the center stage and allowed other races to have their fifteen cosmic minutes of significance. Unwillingly, these peoples were serving their betters' own masterplan - every little discovery, every leap of knowledge, every colony and every starship was infiltrated, proded, mined and ultimately discarded while the Shadows rebuilt their capabilities. And whenever it seemed that one of those young upstarts was about to grow enough in power and understanding so as to able to perceive and even challenge them, the Forever Shadows took the necessary steps to guarantee that only one master race would prevail. Sudden rebellions and schisms tended to happen at this stage of civilization development, plaguing every sentient species that ever reached the stars.

Honestly, it was their own fault they were so prone to fratricide. And gullible, too. It was so easy to introduce division and distrust and throw them at each throats... Uranites and Gaians, Volmorians and Radexians, Planetars and Solarians, Virtual and Concrete... They were all itching for a fight, anyway. The Forever Shadows simply... gave a friendly push.

And so they have always been, watching, skimming, manipulating... But their long game is almost done, and the time draws near when the fledging empires of the galaxy will finally discover who they have been truly serving all this time. And then they will bow. Or be bent. It's their fate.

[And if it ever seems hopeless to fight parasitic abominations who are literally older than time and have the combined knowledge of the Endless and their Lost Ones predecessors at their fingertips... Well, do remember that misinformation is a spy's greatest weapon, hmm?]