It started with "Let's Go, Brandon" meme. Which was when a NASCAR camera captured the chant, "Fuck Joe Biden" and the announcer played it off as "Let's go Brandon". This was facile and seen through by everyone instantly. But the right-wing in America picked up "Let's go Brandon" as a way to say "Fuck Joe Biden" without swearing or being overt. A base subterfuge that the right wing thinks is clever and obscure.

Sometime after that the Dark Brandon meme appeared which is a highly ironic meme usually picturing Joe Biden wearing his aviator sunglasses and smiling. It depicts a "darker, edgier" Joe Biden (doesn't take much) usually in concert with a political victory or getting something done. This can exist because Joe Biden is generally seen as so very boring that any amount of aggression or elevated emotional state would be seen as "extreme".

So now most people use Dark Brandon as a joke to try to trigger the far right or just as a tongue-in-cheek reference to Biden being kinda boring.