Maksim Chmerkovskiy -- The Kate Upton Mystery Solved

We now know what in the Hell Kate Upton was doing hanging out with "Dancing With the Stars" dude Maksim Chmerkovskiy -- and we're still not happy about it.

As we previously (and jealously) reported ... Maks and Kate were spotted leaving a restaurant together in NYC last week.

So, of course we've been wondering how someone known as the "bad boy of the ballroom" ... managed to bag the hottest chick on the planet.

Well, we ran into Maks at LAX, sans Kate, and asked if they're, y'know ... banging?