Vince Vaughn, JB Smoove, Julius Randle Roll Up To Cosm L.A. Opening For UFC 303

Hollywood stars, sports legends, and a billionaire or two had a kick-ass view for UFC 303, but none of them were at T-Mobile Arena ... instead, Vince Vaughn and a gang of other celebs partied and watched fights at a new, super cool immersive theater in L.A.

Cosm Los Angeles -- a 65,000 square foot theatre with a massive curved LED screen in Hollywood Park (next to the Rams' SoFi Stadium) -- is designed to transport fans, giving them the best seat in the house at the biggest sporting events and concerts.

Think of being in the Octagon as Alex Pereira throws punches and kicks (except you never get hit 😰). That's what it's like ... and thanks to a bunch of partnerships with leagues, promotions, and networks, Cosm will show the UFC, NBA, NHL, Premiere League soccer, USA Soccer and much more.

So, big screen, big partners ... and big stars in attendance.

Aside from the beloved "Swingers" actor, celebs like Ronnie Lott, Julius Randle, Robert Horry, James Worthy, JB Smoove, Kel Mitchell, Stan Kroenke, and others rolled up to the debut event.

As for the action inside the cage ... it was a wild night!

Dan Ige stepped in on two hours (!!!) notice to fight Diego Lopes ... after Brian Ortega was forced to withdraw after waking up sick Saturday morning. After his fever failed to break, it was clear T City couldn't take the Octagon. Lopes won by a unanimous decision -- two rounds to one -- but Ige showed himself to be an absolute beast (and so did Lopes).

In the main event -- originally slated to be Conor McGregor-Michael Chandler before The Notorious backed out with an injury last month -- Poatan dropped Jiri Prochazka with a devastating and perfectly placed head kick early in the second round, before finishing the Czech fighter with nasty ground and pound.