The actress/singer made a big entrance Monday at the Metropolitan Museum of Art in NYC -- where fashion's biggest night is underway ... and where J Lo, a co-chair for the event, rolled up by herself, without her hubby by her side.

No need to panic though -- not only was Jen wearing a wedding band on that finger ... but we've learned there's a perfectly good explanation -- the dude is literally filming right now.

BA's rep tells TMZ ... Ben is in Los Angeles at the moment and was never planning on attending the Met Gala with J Lo. We're told he's filming "The Accountant 2," which has been in production since March ... and for which Ben continues to maintain a clean-shaven face.

In other words ... it's all going according to plan. And if you look at what Jen was doing last year at this very same event ... not much has changed, 'cause Ben wasn't there either.

You'll recall ... JL walked up the stairs in 2023 by herself as well. They didn't hit the event in 2022 -- the year they got hitched. On its face, it seems it just isn't Ben's scene ... but it is his best friend's -- Matt Damon is in the building and he's there with his own wife, smiling.

Remember ... Ben and Jen were opposite coasts this weekend too -- she was in New York getting ready for the gala, attending a pre-event dinner where she had some car trouble.

Ben, meanwhile, went to the Tom Brady roast ... where he delivered a few scathing digs.

His set at the mic was met with mixed reviews ... some felt it wasn't really all that funny -- especially when he took aim at internet trolls who talk crap online, calling them a "bitch."

Anyway ... dude's busy on a Monday, and there's no time for posing for cameras with wifey.