Maybe give balboa.io a try, it's the closest I can think of. Not just DMs but also file storage and transfer. Kind of similar UI to slack. Also, peerio.com,

Maybe give balboa.io a try, it's the closest I can think of. Not just DMs but also file storage and transfer. Kind of similar UI to slack. Also, peerio.com, but they don't have a 'channels-like' interface, so maybe not what you're looking for.