DaBaby and Crew Attack DaniLeigh's Brother at Bowling Alley

There's clearly residual underlying beef between DaBaby and DaniLeigh's brother ... evidenced in an all-out brawl between the two at a CA bowling alley -- and now cops are on the case.

Check out the video, obtained by TMZ ... you see DaBaby and Brandon Bills exchange words. Then, DaBaby seems to throw the first punch. The 2 men go at it, and things spill into the bowling lanes as DaBaby's crew gets involved.

It's hard for anyone to stay on their feet during the fight because of the slippery surface, but Brandon Bills is clearly outnumbered and takes an L.

Law enforcement sources say they're now investigating DaBaby for assault with a deadly weapon. We're told it's an ADW case because Bills was kicked in the head while he was already on the ground.

DaBaby and DaniLeigh have a daughter together -- born in July -- and while things haven't been rosy between them ... recently they seemed to lower the temp so they could co-parent.

However, things clearly aren't OK between DaBaby and Brandon, and after the fight, we're told BB stuck around -- shirtless with blood on his face. We're told he refused medical treatment, and DaBaby left before cops arrived.

It was back in November when DaBaby and Dani got in a blowout fight in North Carolina -- she was arrested on 2 counts of assault.

We got Brandon Bills out in L.A. shortly after that incident and he told us if he and DaBaby got in the ring, it'd be a guaranteed first-round knockout.