Note: This backstory may connect to Bowser’s Inside Story happening in this Universe.
Dark Bowser was created by the Dark Star as it was absorbing Bowser’s DNA in order to create a replica of Bowser. But since Dark Bowser flows in dark energy. He takes form with red eyes, blue skin and longer hair instead of normally looking like Bowser.
Bowser had to fight him in a battle in order to defeat the Dark Star. Dark Bowser was defeated but didn’t die, he lived on for longer hidden somewhere. Soon enough, he encounters Mario and his gang on Yoshi’s Island.