Well conservatives were Chanting "Fuck Joe Biden" and the news presenter said they were saying "let's go Brandon" which because a big meme for conservatives. Liberals put glowing red eyes on Biden and called him Dark Brandon.

At a Nascar race there was a reporter interviewing one of the racers. All the people in the audience in the background decided to start shouting "Fuck Biden". The reporter, trying to salvage the interview, insinuated that they were shouting "Let's go Brandon", who I believe was the racer they were interviewing. Conservatives decided that "Let's go Brandon" was now code for "Fuck Biden" because they have a persecution complex and believe they'd be lynched if they actually said it for realsy-reals. Eventually, when Biden started making fun of conservatives, people started styling him as "Dark Brandon" to make him seem cooler.

I hate the term cringe, but there is nothing but when talking about this topic.

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.

Combines the two "let's go Brandon" and "Dark MAGA" both of which are from the political right in US.
Let's go Brandon is described in other replies but...it means fuck Joe Biden
Dark MAGA is being used in the right sphere showing Republican mainly Trump learning from their mistakes and done playing nice... edgy and more aesthetic.

The Brandon thing does come from the place everyone is mentioning, and yes, it's fucking stupid as hell. The "dark" part comes from equally hilarious Chinese propaganda depicting Biden as some kind of evil mastermind dictator. The internet just mashed the two stupid hilarious things together:

https://knowyourmeme.com/memes/dark-brandon

1. a bunch of nascar fans were chanting "fuck joe biden" at a car show
2. a reporter thought they were cheering "let's go brandon" (one of the racecar drivers was named brandon)
3. right wingers start using "let's go brandon" as a minced oath instead of saying "fuck joe biden"
4. left wingers start using "let's go brandon" to signal support for joe biden. this turns into the dark brandon meme

Yes. American politics are that silly.

I’m not sure about the “dark” part, but the “Brandon” comes from when a group of politically right leaning individuals started shouting “F*** Biden” at an event and a news anchor misinterpreted it as “Let’s Go Brandon”.

A reporter was interviewing a race car driver named Brandon at a track following his win in a race. The crowd was chanting a slogan that the reporter mistook to be "Let's Go Brandon", which would make sense, right? It's a race, and Brandon just won? However, for some unknown reason, republicans/rednecks/heathens have decided it makes sense to chant "Fuck Joe Biden" in situations like that. So after this, "Let's Go Brandon" became something degenerates would put on flags, bumper stickers and coffee mugs and so on, as a stand in for "Fuck Joe Biden", demonstrating their high levels of class. Then, someone online made images of Biden looking sorta evil with glowing eyes etc, which were posted in actual appreciation of Biden, sort of making fun of the right-wing claims that he's incompetent and senile. You could read this article for a more thorough explanation plus some examples.

Some folks misreported others shouting fuck Biden as lets go brandon. The right thought this so funny they started branding everything in sight with lets go brandon. Biden supporters created the laser blasting eye Dark Brandon by drawing laser beams shooting from the eyes of the same merch as the lets go brandon folk were pushing. Now Dark Brandon label is strutted out whenever President Joe Biden does something which makes the right's teeth clench.

While the prior is in no way academic nor accurate of detail, it is the best gist of events I may provide.