NOT JUST ANY SEINEN MANGAS.

It's hard to describe because English is not my first language but

I want to find mangas about guys who look kinda like junkies and
"have street cred".

"The atmosphere" should be a little "criminally", "mature" and dark
yet realistic.

Finest examples are:

• Shamo
• Yamikin Ushijima-kun <-- !!!
• Sakitcho Dakedemo

Other not so good examples:

• Oyasumi Bun bun
• Ciguatera
• Boku to Issho
• 17-sai

THE MOST IMPORTANT THING
is that they are not about high schoolers unless it's "one step of their life"
and NOT about yankees or other petty delinquents.
UNLESS they for real like 17-sai.

No mangas like CROWS, WORST or GANG KING where it's all
about showing off the bad boys. Yakuza is ok if it's not the main theme.
Also... SEINEN not shonen. Seinen.

Genre and "topic" does not matter as long as it's interesting
like Shamo and yamikin.

Sorry for being so demanding XDDD

Holyland. Highly recommended.

The world is mine is what you're looking for. Also, maybe Sun-Ken Rock

Sancatuary is pretty good. maybe isn't what you are looking for but is a recommended read

Until Death Do Us Part? some supernatural elements

Koroshiya Ichi
Akumetsu
Shinjuku Swan
Black Joke
Gangsta
Heat
Jackals
Usogui
Dendrobates
Bakune Young

Remembered some more:
Jesus
Freesia
Riman Gambler Mouse
City of Darkness
Killer Stall
Roppongi Black Cross
Dogs - Bullets & Carnage
Jormungand and Black Lagoon

Btw -- some of these are shounen, but they're very seinen-esque, so I mentioned them anyway. And it might also be a good idea to look up seinen manga under categories like Mafia, Underworld Society, Gang/s, Gangster/s, Gang War/s, Yakuza, Loan Shark, Mercenary, Black Market, Underground, etc.

Usogui, smart protagonist involving in gambling.